anchor.diff:        "--enable-avplay"
         "--enable-shared"
         "--enable-runtime-cpudetect"
  +      "--cc=cc"
       ]
         ++ optionals enableGPL [ "--enable-gpl" "--enable-swscale" ]
         ++ optional mp3Support "--enable-libmp3lame"
  
positive.code: 
      outputs = [ "out" "tools" ];
  
      # move avplay to get rid of the SDL dependency in the main output
      postInstall = ''
        mkdir -p "$tools/bin"
        mv "$out/bin/avplay" "$tools/bin"
      '';
  
      doInstallCheck = false; # fails randomly
  
negative.code: { stdenv, fetchurl, pkgconfig, yasm, bzip2, zlib, perl
  , mp3Support    ? true,   lame      ? null
  , speexSupport  ? true,   speex     ? null
  , theoraSupport ? true,   libtheora ? null
  