Do not use garbage-collection for DARWIN ObjC apps

Xcode 5.1 does not support this feature.

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>
(cherry picked from commit 52a9d1929518feab17b81b0a9db7ba45f69d5071)

Change-Id: Ia383e1f9c4ee4ae19ed81cfedb1541399d7546b3
Reviewed-on: http://gerrit.openafs.org/10925
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>