ffmpeg compilation

ffmpeg is easy to compile, following the doc in REF. However, "export PKG_CONFIG_PATH="$HOME/ffmpeg_build/lib/pkgconfig" is required for successfully compile ffmpeg itself. Besides, if ffplay is needed, libsdl-dev package is required to be installed first.

REF: https://trac.ffmpeg.org/wiki/CompilationGuide
http://stackoverflow.com/questions/20422051/ffmpeg-ffplay-binary-not-getting-generated-on-compilation