xwayland: add dependency on xwayland
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.