gtk: cleanup backend dependencies

gtk: cleanup backend dependencies

Make configure detect gtk x11 backend and link libX11 then.  Make
gtk backend specific code properly #ifdef'ed on the GTK_WINDOWING_*
backends at runtime).  Our gtk ui code should build and run fine on
any platform now.

This also fixes the linker failute due to the new XkbGetKeyboard call
added by commit 3158a3482b0093e41f2b2596fba50774ea31ae08.

Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>