Fix exception when Clang is in use.
Traceback (excerpt):
... File "/Users/kfunk/kderoot/craft/bin/BuildSystem/CMakeBuildSystem.py", line 87, in configureOptions options = BuildSystemBase.configureOptions(self) File "/Users/kfunk/kderoot/craft/bin/BuildSystem/BuildSystemBase.py", line 76, in configureOptions defines += " %s" % self.clangOptions() File "/Users/kfunk/kderoot/craft/bin/BuildSystem/CMakeBuildSystem.py", line 208, in clangOptions return out