tools: fix gyp to work on MacOSX without XCode

This issue has already submitted to the upstream in
https://code.google.com/p/gyp/issues/detail?id=477
Use this commit until the upstream is to be fixed.

PR-URL: https://github.com/iojs/io.js/pull/1325


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>