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 3158a348.

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