It looks like a typo, but actually isn't.  In C++ files they are
equivalent, but since VTK has 218 occurrences of __GNUC__ and only
8 of __GNUG__, I think it's more consistent and less confusing to
standardize.

Change-Id: Ic7c5facc57cb0cac9a9055d4b708c3f2776167e3