[Darwin/macosx] Add support for the optional :clang argument
(cherry picked from commit 2dadabebee3a43c02b5cd53cc06e00ca81d00d19)

From: Conrad Kramer <conrad@kramerapps.com>

git-svn-id: http://svn.howett.net/svn/theos/trunk@604 4410221e-0ddf-4ce3-99c0-2db6c0dbc727