Same error (error: reference to 'Rect' is ambiguous) as in Bug 126433, and the same patch fixes the issue.
See bug #152720 for why that patch was reverted.
Bug #152720 resolves the usage of the CF_AVAILABLE macro on OS X <= 10.9. While CF_AVAILABLE does not even exist in the repo now, support for macOS < 10.13 has also been removed. I'm sure there is a regression afterwards. I actually hold the same idea as Philip Chimento in that thread: CoreFoundation.h should not be included there when building WebKitGTK, due to type conflicts.