[gtk] Remove things that aren't present on OS X.

2 of these will return when they're implemented upstream.
[gtk] We don't need anything from xlib.

We also don't need it at link time apparently (tested on Linux).
On Mac OS X, we are using the Quartz backend, so we don't want
xlib there. Nor would we want it on Windows.
[gtk] glib depends on gobject for some symbols.

This addresses a link error on OS X.
[gtk] A temporary hack while we still use gdk_threads_*

This will go away in the future. It is a workaround for a link
error on Mac OS X where we can't have undefined symbols in
our current build system arrangement.
[duim,gtk] Add duim registry entry for Mac OS X.