Detecting clang on OS X is stymied in some configurations by "gcc"
looking sufficiently like gcc. Inspect "cc" instead, and don't
believe it's __GNUC__ unless it's also not __clang__.

While here, if you manage to find and install PackageMaker.app, it
still works on 10.9 for --binary-macpkg.

>From Sevan Janiyan.

(schmonz)