Add configuration for Darwin

Since the new portable mg snapshot depends on clens, here is the needed configuration to get mg building on Darwin.

This was tested on 10.8.5 with: 

    Apple LLVM version 4.2 (clang-425.0.24) (based on LLVM 3.2svn)
    Target: x86_64-apple-darwin12.5.0
    Thread model: posix