anchor.diff:  { lib, stdenv, fetchFromGitHub, cmake, pkg-config, unzip
   , zlib
  -, enablePython ? false, python2Packages
   , enableGtk2 ? false, gtk2
   , enableJPEG ? true, libjpeg
   , enablePNG ? true, libpng
  
positive.code: 
    buildInputs =
         [ zlib ]
      ++ lib.optional enablePython python2Packages.python
      ++ lib.optional enableGtk2 gtk2
      ++ lib.optional enableJPEG libjpeg
      ++ lib.optional enablePNG libpng
  
negative.code:     ++ lib.optional enableEigen eigen
      ++ lib.optionals stdenv.isDarwin [ Cocoa QTKit ]
      ;
  
    nativeBuildInputs = [ cmake pkg-config unzip ];
  
  