When 148197 landed (see https://bugs.webkit.org/show_bug.cgi?id=114353), it broke the GTK+ builds and is also causing me trouble (trying to create a patch now to build WinCairo using cmake). I think adding #if USE(THREADED_SCROLLING) guards makes sense to resolve the problem. Working on a patch now...
Created attachment 197908 [details] Proposed fix
My apologies, I already fixed this but neglected to link it on the original bug! *** This bug has been marked as a duplicate of bug 114525 ***