xwayland: Add a pkg-config file for Xwayland

Xwayland is usually spawned by the Wayland compositor which sets the
command line options.

If a command line option is not supported, Xwayland will fail to start.

That somehow makes the Xwayland command line option sort of ABI, the
Wayland compositor need to know if a particular option is supported by
Xwayland at build time.

Also, currently, Xwayland is being installed along with the rest of the
common executable programs that users may run, which is sub-optimal
because, well, Xwayland is not a common executable program, it's meant
to be a proxy between the Wayland compositor and the legacy X11 clients
which wouldn't be able to run on Wayland otherwise.

Xwayland would be better installed in `libexec` but that directory is
(purposedly) not in the user `PATH` and therefore the Wayland compositor
may not be able to find Xwayland in that case.

To solve both problems (which options are supported by Xwayland and
where to look for it), add a `pkg-config` file specifically for Xwayland
which gives the full path to Xwayland (`xwayland`) and which options it
supports (using `pkg-config` variables).

The `pkg-config` file also provides the `Version` so the build scripts
can check for a particular version if necessary.

Obviously, Wayland compositors are not required to use the `pkg-config`
file and can continue to use whatever mechanism they deem preferable.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
4 jobs for xwayland-pkgconfig in 4 minutes and 40 seconds (queued for 1 second)