==== ffmpeg android build ====
Under linux mint 17.4, you can choose any OS, even cygwin.
   - download android ndk, in this document, I used "android-ndk-r14b"
   - download ffmpeg source, http://www.ffmpeg.org/download
   - unzip ndk
   - make standalone compiler
     * <code>
$NDK/build/tools/make-standalone-toolchian.sh --platform=android-21 --install-dir=~/home/android_toolchain
</code>
     * "android-21" means minimum sdk, 21 is android 6.0 marshmallow
   - make android build script
    * in $FFMPEG directory,
    * <code>
NDK=/home/kjkim/android-ndk-r14b
SYSROOT=$NDK/platforms/android-21/arch-arm
TOOLCHAIN=/home/kjkim/android_toolchain
function build_one
{
./configure \
    --prefix=$PREFIX \
    --enable-shared \
    --disable-static \
    --disable-doc \
    --disable-ffmpeg \
    --disable-ffplay \
    --disable-ffprobe \
    --disable-ffserver \
    --disable-avdevice \
    --disable-doc \
    --disable-symver \
    --cross-prefix=$TOOLCHAIN/bin/arm-linux-androideabi- \
    --target-os=linux \
    --arch=arm \
    --enable-cross-compile \
    --sysroot=$SYSROOT \
    --extra-cflags="-Os -fpic $ADDI_CFLAGS" \
    --extra-ldflags="$ADDI_LDFLAGS" \
    $ADDITIONAL_CONFIGURE_FLAG
make clean
make
make install
}
CPU=arm
PREFIX=$(pwd)/android/$CPU
ADDI_CFLAGS="-marm"
build_one </code>