Check that the pkg-config file is available. This will be required
in the future to check whether xwayland supports features such as
-listenfd, -initfd or -verbose.

If there's no pkg-config file, check that the Xwayland executable
is available.

This effectively makes our relationship with xwayland closer to what
a dynamic library is: checked at build-time, but can be overridden
at run-time.