Implementing building two separate binaries for x11 and wayland

Build gnome-shell for x11, and gnome-shell-wayland for wayland
(as well as the associated libgnome-shell and libgnome-shell-wayland).
The first one links to libmutter, the second to libmutter-wayland.

libgnome-shell and libgnome-shell-wayland are now compiled from
libgnome-shell-base (with all sources that are independent of mutter),
libgnome-shell-menu (with the copy-pasted gtk sources), plus the
sources that use mutter API

https://bugzilla.gnome.org/show_bug.cgi?id=705497