build: make wayland support unconditional

In the wayland branch of mutter, we want to build a wayland version
of the mutter libraries, and that's much easier if we just build
wayland support unconditionally.
The define is kept to avoid a huge diff, but should be removed
in a later patch.
Also, wayland support can still be disable at runtime, by
launching mutter without the --nested switch.

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