anchor.diff:  
     propagatedBuildInputs = with stdenv.lib;
       [ numpy six protobuf3_2 swig werkzeug mock ]
  -    ++ optionals cudaSupport [ cudatoolkit cudnn gcc49 ];
  +    ++ optionals cudaSupport [ cudatoolkit cudnn stdenv.cc ];
   
     # Note that we need to run *after* the fixup phase because the
     # libraries are loaded at runtime. If we run in preFixup then
  
positive.code: , swig
  , werkzeug
  , mock
  , gcc
  , zlib
  }:
  
  assert cudaSupport -> cudatoolkit != null
                     && cudnn != null
                     && gcc49 != null
                     && linuxPackages != null;
  
  # unsupported combination
  
negative.code:   postFixup = let
      rpath = stdenv.lib.makeLibraryPath
        (if cudaSupport then
          [ stdenv.cc.cc.lib zlib cudatoolkit cudnn
            linuxPackages.nvidia_x11 ]
        else
          [ stdenv.cc.cc.lib zlib ]
        );
    in
    ''
  