FindGTK2: Fix missing optional include dirs.

FindGTK2 was adding optional include directories to the
interface include dirs regardless of whether or not they
existed.  This ensures that the directories only get added
if they are actually found. This is particularly a problem
on Solaris where the gtk2 libs and headers might exist but
the FreeType2 headers might not.
Merge topic 'fix-gtk2-missing-optional-includes' into next

268ab7cc FindGTK2: Fix missing optional include dirs.