anchor.diff:  assert vpxSupport -> libvpx != null;
   assert x264Support -> x264 != null;
   assert xvidSupport -> xvidcore != null;
  +assert opusSupport -> libopus != null;
   assert vdpauSupport -> libvdpau != null;
   assert vaapiSupport -> libva != null;
   assert faacSupport -> faac != null;
  
positive.code:     ++ stdenv.lib.optional vpxSupport "--enable-libvpx"
      ++ stdenv.lib.optional x264Support "--enable-libx264"
      ++ stdenv.lib.optional xvidSupport "--enable-libxvid"
      ++ stdenv.lib.optional vdpauSupport "--enable-vdpau"
      ++ stdenv.lib.optional faacSupport "--enable-libfaac --enable-nonfree"
      ++ stdenv.lib.optional dc1394Support "--enable-libdc1394"
  
negative.code:     ++ stdenv.lib.optional vpxSupport libvpx
      ++ stdenv.lib.optional x264Support x264
      ++ stdenv.lib.optional xvidSupport xvidcore
      ++ stdenv.lib.optional opusSupport libopus
      ++ stdenv.lib.optional vdpauSupport libvdpau
      ++ stdenv.lib.optional vaapiSupport libva
      ++ stdenv.lib.optional faacSupport faac
  