This adds a --enable-wayland configure option to enable building mutter
as a hybrid X and Wayland compositor. By default the option is disabled.
If enabled then HAVE_WAYLAND is defined for C code and as an automake
conditional.
This copies the xserver.xml wayland protocol into a protocol/ directory
since wayland support will depend on this protocol for communicating
with an xwayland X server. Copying the spec like this is consistent with
Weston so we don't need a configure option to locate an external spec.
