Fix incorrect system python include path on Mavericks

This fixes an issue regarding incorrect python include path information being passed on to formula under Mavericks. It appears that Mavericks is shipping the Xcode headers under /System/Library/Frameworks/Python.framework again, regardless of whether the CLI tools are installed or not. At the same time there is no longer a Python framework shipped with the Xcode 5 MacOS SDK, as was the case with previous versions.

The modification should partially fix several issues: #23468, https://github.com/Homebrew/homebrew-science/issues/401