Do not use garbage-collection for DARWIN ObjC apps

Xcode 5.1 does not support this feature.

Change-Id: I30457ae73f867637ab154b94d4a6c57089d9b48d
Reviewed-on: http://gerrit.openafs.org/10890
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>