instead of our own variable.
  * ./modules/audio_filter/converter/a52tospdif.c, ./modules/codec/a52.c: added
    <unistd.h>, required for bswap.
  * ./modules/demux/mp4/libmp4.c: removed an unneccessary semicolon in a
    macro definition.
  * ./modules/audio_output/esd.c: first attempt at getting esd to work.
  * ./modules/video_output/*: split sdl.so into a video and an audio plugin,
    and removed useless subdirectories.
  * ./modules/audio_output/sdl.c: compilation fix.