Build Wayland back end with CMake.

The current Wayland CMake code was not functional, as it was not capable of finding the protocols data directory as well as the wayland-scanner. This is now done with just a few lines of pkgconfig CMake calls, which should be available on all Wayland installations.

I also resurrected my old CMake Wayland build which didn't go upstream with the initial release of the compositor. It generates the matching xdg-shell stable protocol header and C file and adds it to the build system, as is currently done in the meson build.

I also removed the unused BUILD_WITH_WAYLAND option, not sure if we care about that. Currently Wayland will be built if available. Btw, BUILD_WITH_XLIB also doesn't seem to be used.