anchor.diff:  assert vaapiSupport -> libva != null;
   assert faacSupport -> faac != null;
   assert x11grabSupport -> libXext != null && libXfixes != null;
  +assert playSupport -> SDL != null;
   
   stdenv.mkDerivation rec {
     name = "ffmpeg-1.1.3";
  
positive.code:     ++ stdenv.lib.optional vaapiSupport libva
      ++ stdenv.lib.optional faacSupport faac
      ++ stdenv.lib.optional dc1394Support libdc1394
      ++ stdenv.lib.optionals x11grabSupport [ libXext libXfixes ];
  
    enableParallelBuilding = true;
      
  
negative.code: , faacSupport ? false, faac ? null
  , dc1394Support ? false, libdc1394 ? null
  , x11grabSupport ? false, libXext ? null, libXfixes ? null
  , playSupport ? true, SDL ? null
  }:
  
  assert speexSupport -> speex != null;
  