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.

README.md

@@ -56,6 +56,7 @@ Install dependencies:
 
 If you choose to enable X11 support:
 
+* xwayland (build-time only, optional at runtime)
 * xcb
 * xcb-composite
 * xcb-xfixes