bash script to build ffmpeg dynamic library with x264 and libfdk-aac for android

build enveroment:
OS:ubuntu 14.04 64bit TLS
NDK: android-ndk-r13b

usage:source set-build.sh [option]"
option can be：
all: cross-compile all components and rebuild libmedia.so
x264: cross-compile x264,ffmpeg and rebuild libmedia.so
libfdk-aac: cross-compile libfdk-aac, ffmpeg and rebuild libmedia.so
ffmpeg: cross-compile ffmpeg and rebuild libmedia.so
help: print help message

FAQ：

1. how to change android api level and API?
   change the values of LIBMEDIA_TARGET_PLATFORM and LIBMEDIA_TARGET_ARCH_ABI in set-build.sh to suitable value. ABI should be equal to the value in TABLE 1 of this page.[url: https://developer.android.com/ndk/guides/abis.html], PLATFORM should be equal to in the target path of $NDK/platforms/

2. which version do i used in my project?
   ffmpeg 3.2.2
   x264 master branch, last update:2017-01-23, https://git.videolan.org/?p=x264.git;a=summary
   libfdk-aac https://github.com/mstorsjo/fdk-aac/releases