xorg.libXt: fix cpp usage under llvm

      sed 's,^as_dummy.*,as_dummy="\$PATH",' -i configure

    '';

    configureFlags = attrs.configureFlags or []

      ++ malloc0ReturnsNullCrossFlag;

      ++ malloc0ReturnsNullCrossFlag

      ++ lib.optional (stdenv.targetPlatform.useLLVM or false) "ac_cv_path_RAWCPP=cpp";

    propagatedBuildInputs = attrs.propagatedBuildInputs or [] ++ [ xorg.libSM ];

    depsBuildBuild = [ buildPackages.stdenv.cc ];

    CPP = if stdenv.isDarwin then "clang -E -" else "${stdenv.cc.targetPrefix}cc -E -";