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.

.builds/archlinux.yml

@@ -13,6 +13,7 @@ packages:
   - xcb-util-image
   - xcb-util-renderutil
   - xcb-util-wm
+  - xorg-xwayland
   - seatd
 sources:
   - https://github.com/swaywm/wlroots