BuildSystem: fix Xcode when building w/ clang targeting 10.6

When using clang (the new project default compiler), disable
implicit objc-runtime linking for 10.6 only.

Before fix, the following error is produced:

Undefined symbols for architecture x86_64:
  "_objc_retain", referenced from:
      ___ARCLite__load in libarclite_macosx.a(arclite.o)
     (maybe you meant: _objc_retainedObject)
ld: symbol(s) not found for architecture x86_64


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5819 b64f7644-9d1e-0410-96f1-a4d463321fa5