anchor.diff:      #"-DLLVM_CONFIG=${llvm}/bin/llvm-config"
       "-DLLVM_BUILD_TESTS=ON"
       "-DCLANG_INCLUDE_TESTS=ON"
  -  ] ++ (stdenv.lib.optional stdenv.isLinux
  +  ] ++ (stdenv.lib.optional (stdenv.isLinux && haveGcc)
       # necessary for clang to find crtend.o
       "-DGCC_INSTALL_PREFIX=${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: 
    passthru = {
      isClang = true;
    } // stdenv.lib.optionalAttrs haveGcc {
      inherit gcc;
    };
  
  