anchor.diff:  , vpxSupport ? false, libvpx ? null
   , x264Support ? true, x264 ? null
   , xvidSupport ? true, xvidcore ? null
  +, opusSupport ? true, libopus ? null
   , vdpauSupport ? true, libvdpau ? null
   , vaapiSupport ? true, libva ? null
   , faacSupport ? false, 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: 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;
  