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 tells where Xwayland is installed (xwaylandpath and 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