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