Only make libdecor a hard-requirement for using Wayland in gnome-shell
when X11 is available, as it's possible to disable Xwayland.
This also fixes window-borders not being used in gnome-shell when
WITH_GHOST_X11 is off.
Now it's possible to test Blender under gnome without libdecor by
uninstalling libdecor & running with DISPLAY environment variable
set to an empty string - useful for troubleshooting issues which
could be caused by libdecor.