Just rebased the rosuav/support-gdk-quartz branch onto current 8.1 (trivial since there'd been no changes to GTK2) so you'll get a forced-update warning when you grab this.
I don't know anything about building GTK2, but there is a change in behaviour in your patch. Before it on Windows gdk/gdkwin32.h would always be included, while now it will only be included if gdk/gdkx.h does not exists.
I would suggest to include all the relevant header files it can find, unless that creates a problem.