suitable clang stdenv

pkgs/development/compilers/llvm/3.3/clang.nix

@@ -1,5 +1,9 @@
 { stdenv, fetchurl, perl, groff, llvm, cmake, libxml2, python }:
 
+# be sure not to rebuild clang on darwin; some packages request it specifically
+# we need to fix those
+assert stdenv.isDarwin -> stdenv.gcc.nativeTools;
+
 let
   version = "3.3";
   gccReal = if (stdenv.gcc.gcc or null) == null then stdenv.gcc else stdenv.gcc.gcc;