Fix broken build with Clang < 3.4 after 60aed669345be33b916c445565

The -Wunused-const-variable option was added in Clang 3.4 and is not yet
supported on Apple's Clang. This test needs to be extended with the
Apple Clang version that supports the option.

Change-Id: Ief6ece91f1c0200c3359e74dafca4b893fcde7ca
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>