anchor.diff:  # PulseAudio.
   assert (stdenv.isLinux && !(stdenv ? cross)) -> alsaSupport || pulseaudioSupport;
   
  -assert openglSupport -> (mesa != null && x11Support);
  +assert openglSupport -> (mesa_noglu != null && x11Support);
   assert x11Support -> (xlibsWrapper != null && libXrandr != null);
   assert alsaSupport -> alsaLib != null;
   assert pulseaudioSupport -> libpulseaudio != null;
  
positive.code:     optionals x11Support [ xlibsWrapper libXrandr ] ++
      optional alsaSupport alsaLib ++
      optional stdenv.isLinux libcap ++
      optional openglSupport mesa ++
      optional pulseaudioSupport libpulseaudio ++
      optional stdenv.isDarwin Cocoa;
  
  
negative.code: { stdenv, fetchurl, fetchpatch, pkgconfig, audiofile, libcap
  , openglSupport ? false, mesa_noglu ? null
  , alsaSupport ? true, alsaLib ? null
  , x11Support ? true, xlibsWrapper ? null, libXrandr ? null
  , pulseaudioSupport ? true, libpulseaudio ? null
  