Instead of separate checkouts of the FFmpeg libavutil/, libavcodec/,
libavformat/, libpostproc/ subdirectories make MPlayer build with a
complete checkout of FFmpeg under ffmpeg/.
configure now creates config.mak and config.h at both the top level
and in ffmpeg/. The config.h files are identical. The config.mak for
FFmpeg has different include paths. The top-level common.mak and
subdir.mak are now used by libswscale only; the directories under
ffmpeg/ use the versions in ffmpeg/.
