Depending on the OS the lib dir can vary, on Fedora for instance it is
"${prefix}/lib64". Instead of hard-coding the directory name, let CMake fill
this variable for us.
- search for ffmpeg libs/headers at cmake time
- added ffmpeg-transcode.cpp into libcommon if ffmpeg on
- hooked ffmpeg trancoding in common read_wav(...)
- passed test:
./main -m ggml-base.en.bin -f samples/jfk.mp3