core: Select the X11 backend of Clutter

Mutter depends on the X11 windowing backend of Clutter, unless it's used
as a Wayland display server.

This allows Mutter to run without breaking in case Clutter changes the
order with which windowing backends are selected, like it was the case
for bug https://bugzilla.gnome.org/show_bug.cgi?id=734587

The order of selection of the Clutter backends has not been made public,
so it cannot be relied upon since the introduction of the multiple
backends support; since Mutter requires the X11 backend functionality,
it should select the X11 windowing system, in the same way it selects
the EGL backend when compiled and run as a Wayland display server.