Add LLVM config headers for OSX.

Change-Id: I08d5d852322a75c8b3a0d6932f229da7022a82f1
Reviewed-on: https://swiftshader-review.googlesource.com/4379
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>