[GTK] Bump the minimum required Clang version to 3.3

https://bugs.webkit.org/show_bug.cgi?id=127178

Reviewed by Martin Robinson.

* Source/autotools/CheckSystemAndBasicDependencies.m4: Require Clang 3.3 or later when using the Clang compiler to
build the GTK port. Building with Clang 3.2 doesn't work anymore and there's no point in fixing that, as we should
support the latest two versions of Clang (3.3 and 3.4), just like we do with GCC (4.7 and 4.8).


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162249 268f45cc-cd09-0410-ab3c-d52691b4dbfc