    Android.mk file syntax specification

    Introduction:
    -------------

    This document describes the syntax of Android.mk build file
    written to describe your C and C++ source files to the Android
    NDK. To understand what follows, it is assumed that you have
    read the docs/OVERVIEW.html file that explains their role and
    usage.

    Overview:
    ---------

    An Android.mk file is written to describe your sources to the
    build system. More specifically:

    - The file is really a tiny GNU Makefile fragment that will be
      parsed one or more times by the build system. As such, you
      should try to minimize the variables you declare there and
      do not assume that anything is not defined during parsing.

    - The file syntax is designed to allow you to group your
      sources into 'modules'. A module is one of the following:

        - a static library
        - a shared library

      Only shared libraries will be installed/copied to your
      application package. Static libraries can be used to generate
      shared libraries though.

      You can define one or more modules in each Android.mk file,
      and you can use the same source file in several modules.

    - The build system handles many details for you. For example, you
      don't need to list header files or explicit dependencies between
      generated files in your Android.mk. The NDK build system will
      compute these automatically for you.

      This also means that, when updating to newer releases of the NDK,
      you should be able to benefit from new toolchain/platform support
      without having to touch your Android.mk files.

    Note that the syntax is *very* close to the one used in Android.mk files
    distributed with the full open-source Android platform sources. While
    the build system implementation that uses them is different, this is
    an intentional design decision made to allow reuse of 'external' libraries'
    source code easier for application developers.

    Simple example:
    ---------------

    Before describing the syntax in details, let's consider the simple
    "hello JNI" example, i.e. the files under:

        apps/hello-jni/project

    Here, we can see:

      - The 'src' directory containing the Java sources for the
        sample Android project.

      - The 'jni' directory containing the native source for
        the sample, i.e. 'jni/hello-jni.c'

        This source file implements a simple shared library that
        implements a native method that returns a string to the
        VM application.

      - The 'jni/Android.mk' file that describes the shared library
        to the NDK build system. Its content is:

       ---------- cut here ------------------
       LOCAL_PATH := $(call my-dir)

       include $(CLEAR_VARS)

       LOCAL_MODULE    := hello-jni
       LOCAL_SRC_FILES := hello-jni.c

       include $(BUILD_SHARED_LIBRARY)
       ---------- cut here ------------------

    Now, let's explain these lines:

      LOCAL_PATH := $(call my-dir)

    An Android.mk file must begin with the definition of the LOCAL_PATH variable.
    It is used to locate source files in the development tree. In this example,
    the macro function 'my-dir', provided by the build system, is used to return
    the path of the current directory (i.e. the directory containing the
    Android.mk file itself).

      include $(CLEAR_VARS)

    The CLEAR_VARS variable is provided by the build system and points to a
    special GNU Makefile that will clear many LOCAL_XXX variables for you
    (e.g. LOCAL_MODULE, LOCAL_SRC_FILES, LOCAL_STATIC_LIBRARIES, etc...),
    with the exception of LOCAL_PATH. This is needed because all build
    control files are parsed in a single GNU Make execution context where
    all variables are global.

      LOCAL_MODULE := hello-jni

    The LOCAL_MODULE variable must be defined to identify each module you
    describe in your Android.mk. The name must be *unique* and not contain
    any spaces. Note that the build system will automatically add proper
    prefix and suffix to the corresponding generated file. In other words,
    a shared library module named 'foo' will generate 'libfoo.so'.

    IMPORTANT NOTE:
    If you name your module 'libfoo', the build system will not
    add another 'lib' prefix and will generate libfoo.so as well.
    This is to support Android.mk files that originate from the
    Android platform sources, would you need to use these.

      LOCAL_SRC_FILES := hello-jni.c

    The LOCAL_SRC_FILES variables must contain a list of C and/or C++ source
    files that will be built and assembled into a module. Note that you should
    not list header and included files here, because the build system will
    compute dependencies automatically for you; just list the source files
    that will be passed directly to a compiler, and you should be good.

    Note that the default extension for C++ source files is '.cpp'. It is
    however possible to specify a different one by defining the variable
    LOCAL_CPP_EXTENSION. Don't forget the initial dot (i.e. '.cxx' will
    work, but not 'cxx').

      include $(BUILD_SHARED_LIBRARY)

    The BUILD_SHARED_LIBRARY is a variable provided by the build system that
    points to a GNU Makefile script that is in charge of collecting all the
    information you defined in LOCAL_XXX variables since the latest
    'include $(CLEAR_VARS)' and determine what to build, and how to do it
    exactly. There is also BUILD_STATIC_LIBRARY to generate a static library.

    There are more complex examples in the samples directories, with commented
    Android.mk files that you can look at.

    Reference:
    ----------

    This is the list of variables you should either rely on or define in
    an Android.mk. You can define other variables for your own usage, but
    the NDK build system reserves the following variable names:

    - names that begin with LOCAL_  (e.g. LOCAL_MODULE)
    - names that begin with PRIVATE_, NDK_ or APP_  (used internally)
    - lower-case names (used internally, e.g. 'my-dir')

    If you need to define your own convenience variables in an Android.mk
    file, we recommend using the MY_ prefix, for a trivial example:

       ---------- cut here ------------------
        MY_SOURCES := foo.c
        ifneq ($(MY_CONFIG_BAR),)
          MY_SOURCES += bar.c
        endif

        LOCAL_SRC_FILES += $(MY_SOURCES)
       ---------- cut here ------------------

    So, here we go:

    NDK-provided variables:
    - - - - - - - - - - - -

    These GNU Make variables are defined by the build system before
    your Android.mk file is parsed. Note that under certain circumstances
    the NDK might parse your Android.mk several times, each with different
    definition for some of these variables.

    CLEAR_VARS
        Points to a build script that undefines nearly all LOCAL_XXX variables
        listed in the "Module-description" section below. You must include
        the script before starting a new module, e.g.:

          include $(CLEAR_VARS)

    BUILD_SHARED_LIBRARY
        Points to a build script that collects all the information about the
        module you provided in LOCAL_XXX variables and determines how to build
        a target shared library from the sources you listed. Note that you
        must have LOCAL_MODULE and LOCAL_SRC_FILES defined, at a minimum before
        including this file. Example usage:

          include $(BUILD_SHARED_LIBRARY)

        note that this will generate a file named lib$(LOCAL_MODULE).so

    BUILD_STATIC_LIBRARY
        A variant of BUILD_SHARED_LIBRARY that is used to build a target static
        library instead. Static libraries are not copied into your
        project/packages but can be used to build shared libraries (see
        LOCAL_STATIC_LIBRARIES and LOCAL_WHOLE_STATIC_LIBRARIES described below).
        Example usage:

          include $(BUILD_STATIC_LIBRARY)

        Note that this will generate a file named lib$(LOCAL_MODULE).a

    PREBUILT_SHARED_LIBRARY
        Points to a build script used to specify a prebuilt shared library.
        Unlike BUILD_SHARED_LIBRARY and BUILD_STATIC_LIBRARY, the value
        of LOCAL_SRC_FILES must be a single path to a prebuilt shared
        library (e.g. foo/libfoo.so), instead of a source file.

        You can reference the prebuilt library in another module using
        the LOCAL_PREBUILTS variable (see docs/PREBUILTS.html for more
        information).

    PREBUILT_STATIC_LIBRARY
        This is the same as PREBUILT_SHARED_LIBRARY, but for a static library
        file instead. See docs/PREBUILTS.html for more.

    TARGET_ARCH
        Name of the target CPU architecture as it is specified by the
        full Android open-source build. This is 'arm' for any ARM-compatible
        build, independent of the CPU architecture revision.

    TARGET_PLATFORM
        Name of the target Android platform when this Android.mk is parsed.
        For example, 'android-3' correspond to Android 1.5 system images. For
        a complete list of platform names and corresponding Android system
        images, read docs/STABLE-APIS.html.

    TARGET_ARCH_ABI
        Name of the target CPU+ABI when this Android.mk is parsed.
        Two values are supported at the moment:

           armeabi
                For ARMv5TE

           armeabi-v7a

        NOTE: Up to Android NDK 1.6_r1, this variable was simply defined
              as 'arm'. However, the value has been redefined to better
              match what is used internally by the Android platform.

        For more details about architecture ABIs and corresponding
        compatibility issues, please read docs/CPU-ARCH-ABIS.html

        Other target ABIs will be introduced in future releases of the NDK
        and will have a different name. Note that all ARM-based ABIs will
        have 'TARGET_ARCH' defined to 'arm', but may have different
        'TARGET_ARCH_ABI'

    TARGET_ABI
        The concatenation of target platform and ABI, it really is defined
        as $(TARGET_PLATFORM)-$(TARGET_ARCH_ABI) and is useful when you want
        to test against a specific target system image for a real device.

        By default, this will be 'android-3-armeabi'

        (Up to Android NDK 1.6_r1, this used to be 'android-3-arm' by default)

    NDK-provided function macros:
    - - - - - - - - - - - - - - -

    The following are GNU Make 'function' macros, and must be evaluated
    by using '$(call <function>)'. They return textual information.

    my-dir
        Returns the path of the last included Makefile, which typically is
        the current Android.mk's directory. This is useful to define
        LOCAL_PATH at the start of your Android.mk as with:

            LOCAL_PATH := $(call my-dir)

        IMPORTANT NOTE: Due to the way GNU Make works, this really returns
        the path of the *last* *included* *Makefile* during the parsing of
        build scripts. Do not call my-dir after including another file.

        For example, consider the following example:

            LOCAL_PATH := $(call my-dir)

            ... declare one module

            include $(LOCAL_PATH)/foo/Android.mk

            LOCAL_PATH := $(call my-dir)

            ... declare another module

        The problem here is that the second call to 'my-dir' will define
        LOCAL_PATH to $PATH/foo instead of $PATH, due to the include that
        was performed before that.

        For this reason, it's better to put additional includes after
        everything else in an Android.mk, as in:

            LOCAL_PATH := $(call my-dir)

            ... declare one module

            LOCAL_PATH := $(call my-dir)

            ... declare another module

            # extra includes at the end of the Android.mk
            include $(LOCAL_PATH)/foo/Android.mk

        If this is not convenient, save the value of the first my-dir call
        into another variable, for example:

            MY_LOCAL_PATH := $(call my-dir)

            LOCAL_PATH := $(MY_LOCAL_PATH)

            ... declare one module

            include $(LOCAL_PATH)/foo/Android.mk

            LOCAL_PATH := $(MY_LOCAL_PATH)

            ... declare another module

    all-subdir-makefiles
        Returns a list of Android.mk located in all sub-directories of
        the current 'my-dir' path. For example, consider the following
        hierarchy:

            sources/foo/Android.mk
            sources/foo/lib1/Android.mk
            sources/foo/lib2/Android.mk

        If sources/foo/Android.mk contains the single line:

            include $(call all-subdir-makefiles)

        Then it will include automatically sources/foo/lib1/Android.mk and
        sources/foo/lib2/Android.mk

        This function can be used to provide deep-nested source directory
        hierarchies to the build system. Note that by default, the NDK
        will only look for files in sources/*/Android.mk

    this-makefile
        Returns the path of the current Makefile (i.e. where the function
        is called).

    parent-makefile
        Returns the path of the parent Makefile in the inclusion tree,
        i.e. the path of the Makefile that included the current one.

    grand-parent-makefile
        Guess what...

    import-module
        A function that allows you to find and include the Android.mk
        of another module by name. A typical example is:

          $(call import-module,<name>)

        And this will look for the module tagged <name> in the list of
        directories referenced by your NDK_MODULE_PATH environment
        variable, and include its Android.mk automatically for you.

        Read docs/IMPORT-MODULE.html for more details.

    Module-description variables:
    - - - - - - - - - - - - - - -

    The following variables are used to describe your module to the build
    system. You should define some of them between an 'include $(CLEAR_VARS)'
    and an 'include $(BUILD_XXXXX)'. As written previously, $(CLEAR_VARS) is
    a script that will undefine/clear all of these variables, unless explicitly
    noted in their description.

    LOCAL_PATH
        This variable is used to give the path of the current file.
        You MUST define it at the start of your Android.mk, which can
        be done with:

          LOCAL_PATH := $(call my-dir)

        This variable is *not* cleared by $(CLEAR_VARS) so only one
        definition per Android.mk is needed (in case you define several
        modules in a single file).

    LOCAL_MODULE
        This is the name of your module. It must be unique among all
        module names, and shall not contain any space. You MUST define
        it before including any $(BUILD_XXXX) script.

        By default, the module name determines the name of generated files,
        e.g. lib<foo>.so for a shared library module named <foo>. However
        you should only refer to other modules with their 'normal'
        name (e.g. <foo>) in your NDK build files (either Android.mk
        or Application.mk)

        You can override this default with LOCAL_MODULE_FILENAME (see below)

    LOCAL_MODULE_FILENAME
        This variable is optional, and allows you to redefine the name of
        generated files. By default, module <foo> will always generate a
        static library named lib<foo>.a or a shared library named lib<foo>.so,
        which are standard Unix conventions.

        You can override this by defining LOCAL_MODULE_FILENAME, For example:

            LOCAL_MODULE := foo-version-1
            LOCAL_MODULE_FILENAME := libfoo

        NOTE: You should not put a path or file extension in your
        LOCAL_MODULE_FILENAME, these will be handled automatically by the
        build system.

    LOCAL_SRC_FILES
        This is a list of source files that will be built for your module.
        Only list the files that will be passed to a compiler, since the
        build system automatically computes dependencies for you.

        Note that source files names are all relative to LOCAL_PATH and
        you can use path components, e.g.:

          LOCAL_SRC_FILES := foo.c \
                             toto/bar.c

        NOTE: Always use Unix-style forward slashes (/) in build files.
              Windows-style back-slashes will not be handled properly.

    LOCAL_CPP_EXTENSION
        This is an optional variable that can be defined to indicate
        the file extension(s) of C++ source files. They must begin with a dot.
        The default is '.cpp' but you can change it. For example:

            LOCAL_CPP_EXTENSION := .cxx

        Since NDK r7, you can list several extensions in this variable, as in:

            LOCAL_CPP_EXTENSION := .cxx .cpp .cc

    LOCAL_CPP_FEATURES
        This is an optional variable that can be defined to indicate
        that your code relies on specific C++ features. To indicate that
        your code uses RTTI (RunTime Type Information), use the following:

            LOCAL_CPP_FEATURES := rtti

        To indicate that your code uses C++ exceptions, use:

            LOCAL_CPP_FEATURES := exceptions

        You can also use both of them with (order is not important):

            LOCAL_CPP_FEATURES := rtti features

        The effect of this variable is to enable the right compiler/linker
        flags when building your modules from sources. For prebuilt binaries,
        this also helps declare which features the binary relies on to ensure
        the final link works correctly.

        It is recommended to use this variable instead of enabling -frtti and
        -fexceptions directly in your LOCAL_CPPFLAGS definition.

    LOCAL_C_INCLUDES
        An optional list of paths, relative to the NDK *root* directory,
        which will be appended to the include search path when compiling
        all sources (C, C++ and Assembly). For example:

            LOCAL_C_INCLUDES := sources/foo

        Or even:

            LOCAL_C_INCLUDES := $(LOCAL_PATH)/../foo

        These are placed before any corresponding inclusion flag in
        LOCAL_CFLAGS / LOCAL_CPPFLAGS

        The LOCAL_C_INCLUDES path are also used automatically when
        launching native debugging with ndk-gdb.

    LOCAL_CFLAGS
        An optional set of compiler flags that will be passed when building
        C *and* C++ source files.

        This can be useful to specify additional macro definitions or
        compile options.

        IMPORTANT: Try not to change the optimization/debugging level in
                   your Android.mk, this can be handled automatically for
                   you by specifying the appropriate information in
                   your Application.mk, and will let the NDK generate
                   useful data files used during debugging.

        NOTE: In android-ndk-1.5_r1, the corresponding flags only applied
              to C source files, not C++ ones. This has been corrected to
              match the full Android build system behaviour. (You can use
              LOCAL_CPPFLAGS to specify flags for C++ sources only now).

        It is possible to specify additional include paths with
        LOCAL_CFLAGS += -I<path>, however, it is better to use LOCAL_C_INCLUDES
        for this, since the paths will then also be used during native
        debugging with ndk-gdb.

    LOCAL_CXXFLAGS
        An alias for LOCAL_CPPFLAGS. Note that use of this flag is obsolete
        as it may disappear in future releases of the NDK.

    LOCAL_CPPFLAGS
        An optional set of compiler flags that will be passed when building
        C++ source files *only*. They will appear after the LOCAL_CFLAGS
        on the compiler's command-line.

        NOTE: In android-ndk-1.5_r1, the corresponding flags applied to
              both C and C++ sources. This has been corrected to match the
              full Android build system. (You can use LOCAL_CFLAGS to specify
              flags for both C and C++ sources now).

    LOCAL_STATIC_LIBRARIES
        The list of static libraries modules (built with BUILD_STATIC_LIBRARY)
        that should be linked to this module. This only makes sense in
        shared library modules.

    LOCAL_SHARED_LIBRARIES
        The list of shared libraries *modules* this module depends on at runtime.
        This is necessary at link time and to embed the corresponding information
        in the generated file.

    LOCAL_WHOLE_STATIC_LIBRARIES
        A variant of LOCAL_STATIC_LIBRARIES used to express that the corresponding
        library module should be used as "whole archives" to the linker. See the
        GNU linker's documentation for the --whole-archive flag.

        This is generally useful when there are circular dependencies between
        several static libraries. Note that when used to build a shared library,
        this will force all object files from your whole static libraries to be
        added to the final binary. This is not true when generating executables
        though.

    LOCAL_LDLIBS
        The list of additional linker flags to be used when building your
        module. This is useful to pass the name of specific system libraries
        with the "-l" prefix. For example, the following will tell the linker
        to generate a module that links to /system/lib/libz.so at load time:

          LOCAL_LDLIBS := -lz

        See docs/STABLE-APIS.html for the list of exposed system libraries you
        can linked against with this NDK release.

    LOCAL_ALLOW_UNDEFINED_SYMBOLS
        By default, any undefined reference encountered when trying to build
        a shared library will result in an "undefined symbol" error. This is a
        great help to catch bugs in your source code.

        However, if for some reason you need to disable this check, set this
        variable to 'true'. Note that the corresponding shared library may fail
        to load at runtime.

    LOCAL_ARM_MODE
        By default, ARM target binaries will be generated in 'thumb' mode, where
        each instruction are 16-bit wide. You can define this variable to 'arm'
        if you want to force the generation of the module's object files in
        'arm' (32-bit instructions) mode. E.g.:

          LOCAL_ARM_MODE := arm

        Note that you can also instruct the build system to only build specific
        sources in ARM mode by appending an '.arm' suffix to its source file
        name. For example, with:

           LOCAL_SRC_FILES := foo.c bar.c.arm

        Tells the build system to always compile 'bar.c' in ARM mode, and to
        build foo.c according to the value of LOCAL_ARM_MODE.

        NOTE: Setting APP_OPTIM to 'debug' in your Application.mk will also force
              the generation of ARM binaries as well. This is due to bugs in the
              toolchain debugger that don't deal too well with thumb code.

    LOCAL_ARM_NEON
        Defining this variable to 'true' allows the use of ARM Advanced SIMD
        (a.k.a. NEON) GCC intrinsics in your C and C++ sources, as well as
        NEON instructions in Assembly files.

        You should only define it when targeting the 'armeabi-v7a' ABI that
        corresponds to the ARMv7 instruction set. Note that not all ARMv7
        based CPUs support the NEON instruction set extensions and that you
        should perform runtime detection to be able to use this code at runtime
        safely. To learn more about this, please read the documentation at
        docs/CPU-ARM-NEON.html and docs/CPU-FEATURES.html.

        Alternatively, you can also specify that only specific source files
        may be compiled with NEON support by using the '.neon' suffix, as
        in:

            LOCAL_SRC_FILES = foo.c.neon bar.c zoo.c.arm.neon

        In this example, 'foo.c' will be compiled in thumb+neon mode,
        'bar.c' will be compiled in 'thumb' mode, and 'zoo.c' will be
        compiled in 'arm+neon' mode.

        Note that the '.neon' suffix must appear after the '.arm' suffix
        if you use both (i.e. foo.c.arm.neon works, but not foo.c.neon.arm !)

    LOCAL_DISABLE_NO_EXECUTE
        Android NDK r4 added support for the "NX bit" security feature.
        It is enabled by default, but you can disable it if you *really*
        need to by setting this variable to 'true'.

        NOTE: This feature does not modify the ABI and is only enabled on
              kernels targeting ARMv6+ CPU devices. Machine code generated
              with this feature enabled will run unmodified on devices
              running earlier CPU architectures.

        For more information, see:

            http://en.wikipedia.org/wiki/NX_bit
            http://www.gentoo.org/proj/en/hardened/gnu-stack.xml

    LOCAL_DISABLE_RELRO
        By default, NDK compiled code is built with read-only relocations
        and GOT protection.  This instructs the runtime linker to mark
        certain regions of memory as being read-only after relocation,
        making certain security exploits (such as GOT overwrites) harder
        to perform.

        It is enabled by default, but you can disable it if you *really*
        need to by setting this variable to 'true'.

        NOTE: These protections are only effective on newer Android devices
              ("Jelly Bean" and beyond). The code will still run on older
              versions (albeit without memory protections).

        For more information, see:

            http://isisblogs.poly.edu/2011/06/01/relro-relocation-read-only/
            http://www.akkadia.org/drepper/nonselsec.pdf (section 6)

    LOCAL_EXPORT_CFLAGS
        Define this variable to record a set of C/C++ compiler flags that will
        be added to the LOCAL_CFLAGS definition of any other module that uses
        this one with LOCAL_STATIC_LIBRARIES or LOCAL_SHARED_LIBRARIES.

        For example, consider the module 'foo' with the following definition:

            include $(CLEAR_VARS)
            LOCAL_MODULE := foo
            LOCAL_SRC_FILES := foo/foo.c
            LOCAL_EXPORT_CFLAGS := -DFOO=1
            include $(BUILD_STATIC_LIBRARY)

        And another module, named 'bar' that depends on it as:

            include $(CLEAR_VARS)
            LOCAL_MODULE := bar
            LOCAL_SRC_FILES := bar.c
            LOCAL_CFLAGS := -DBAR=2
            LOCAL_STATIC_LIBRARIES := foo
            include $(BUILD_SHARED_LIBRARY)

        Then, the flags '-DFOO=1 -DBAR=2' will be passed to the compiler when
        building bar.c

        Exported flags are prepended to your module's LOCAL_CFLAGS so you can
        easily override them. They are also transitive: if 'zoo' depends on
        'bar' which depends on 'foo', then 'zoo' will also inherit all flags
        exported by 'foo'.

        Finally, exported flags are *not* used when building the module that
        exports them. In the above example, -DFOO=1 would not be passed to the
        compiler when building foo/foo.c.

    LOCAL_EXPORT_CPPFLAGS
        Same as LOCAL_EXPORT_CFLAGS, but for C++ flags only.

    LOCAL_EXPORT_C_INCLUDES
        Same as LOCAL_EXPORT_CFLAGS, but for C include paths.
        This can be useful if 'bar.c' wants to include headers
        that are provided by module 'foo'.

    LOCAL_EXPORT_LDLIBS
        Same as LOCAL_EXPORT_CFLAGS, but for linker flags. Note that the
        imported linker flags will be appended to your module's LOCAL_LDLIBS
        though, due to the way Unix linkers work.

        This is typically useful when module 'foo' is a static library and has
        code that depends on a system library. LOCAL_EXPORT_LDLIBS can then be
        used to export the dependency. For example:

            include $(CLEAR_VARS)
            LOCAL_MODULE := foo
            LOCAL_SRC_FILES := foo/foo.c
            LOCAL_EXPORT_LDLIBS := -llog
            include $(BUILD_STATIC_LIBRARY)

            include $(CLEAR_VARS)
            LOCAL_MODULE := bar
            LOCAL_SRC_FILES := bar.c
            LOCAL_STATIC_LIBRARIES := foo
            include $(BUILD_SHARED_LIBRARY)

        There, libbar.so will be built with a -llog at the end of the linker
        command to indicate that it depends on the system logging library,
        because it depends on 'foo'.

    LOCAL_SHORT_COMMANDS
        Set this variable to 'true' when your module has a very high number of
        sources and/or dependent static or shared libraries. This forces the
        build system to use an intermediate list file, and use it with the
        library archiver or static linker with the @$(listfile) syntax.

        This can be useful on Windows, where the command-line only accepts
        a maximum of 8191 characters, which can be too small for complex
        projects.

        This also impacts the compilation of individual source files, placing
        nearly all compiler flags inside list files too.

        Note that any other value than 'true' will revert to the default
        behaviour. You can also define APP_SHORT_COMMANDS in your
        Application.mk to force this behaviour for all modules in your
        project.

        NOTE: We do not recommend enabling this feature by default, since it
              makes the build slower.

    LOCAL_FILTER_ASM
        Define this variable to a shell command that will be used to filter
        the assembly files from, or generated from, your LOCAL_SRC_FILES.

        When it is defined, the following happens:

          - Any C or C++ source file is generated into a temporary assembly
            file (instead of being compiled into an object file).

          - Any temporary assembly file, and any assembly file listed in
            LOCAL_SRC_FILES is sent through the LOCAL_FILTER_ASM command
            to generate _another_ temporary assembly file.

          - These filtered assembly files are compiled into object file.

        In other words, If you have:

          LOCAL_SRC_FILES  := foo.c bar.S
          LOCAL_FILTER_ASM := myasmfilter

        foo.c --1--> $OBJS_DIR/foo.S.original --2--> $OBJS_DIR/foo.S --3--> $OBJS_DIR/foo.o
        bar.S                                 --2--> $OBJS_DIR/bar.S --3--> $OBJS_DIR/bar.o

        Were "1" corresponds to the compiler, "2" to the filter, and "3" to the
        assembler. The filter must be a standalone shell command that takes the
        name of the input file as its first argument, and the name of the output
        file as the second one, as in:

            myasmfilter $OBJS_DIR/foo.S.original $OBJS_DIR/foo.S
            myasmfilter bar.S $OBJS_DIR/bar.S

    NDK_TOOLCHAIN_VERSION
        Define this variable to either 4.4.3 or 4.6 to select version of GCC compiler.
        4.6 is the default