anchor.diff:  { stdenv, fetchurl, pkgconfig, yasm, zlib, bzip2, alsaLib
  -, jackSupport ? false, jackaudio ? null
   , mp3Support ? true, lame ? null
   , speexSupport ? true, speex ? null
   , theoraSupport ? true, libtheora ? null
  
positive.code: , x11grabSupport ? false, libXext ? null, libXfixes ? null
  }:
  
  assert jackSupport -> jackaudio != null;
  assert speexSupport -> speex != null;
  assert theoraSupport -> libtheora != null;
  assert vorbisSupport -> libvorbis != null;
  
negative.code:     ++ stdenv.lib.optional x11grabSupport "--enable-x11grab";
  
    buildInputs = [ pkgconfig lame yasm zlib bzip2 alsaLib ]
      ++ stdenv.lib.optional mp3Support lame
      ++ stdenv.lib.optional speexSupport speex
      ++ stdenv.lib.optional theoraSupport libtheora
  