anchor.diff:  
     passthru = {
       isClang = true;
  +  } // stdenv.lib.optionalAttrs haveGcc {
       inherit gcc;
     };
   
  
positive.code: 
  let
    rev = emscriptenVersion;
    gcc = if stdenv.cc.isGNU then stdenv.cc.cc else stdenv.cc.cc.gcc;
  in
  stdenv.mkDerivation rec {
  
negative.code:     #"-DLLVM_CONFIG=${llvm}/bin/llvm-config"
      "-DLLVM_BUILD_TESTS=ON"
      "-DCLANG_INCLUDE_TESTS=ON"
    ] ++ (stdenv.lib.optional (stdenv.isLinux && haveGcc)
      # necessary for clang to find crtend.o
      "-DGCC_INSTALL_PREFIX=${gcc}"
    );
  