Instead of walking PATH like a previous proposal [1], this one
checks that the Xwayland path specified in the pkg-config file
exists.
I think this is a reasonable compromise:
- Users that don't have Xwayland installed system-wide won't get
a bogus DISPLAY env variable set up.
- Users that have WLR_XWAYLAND set won't be affected by this check.
- Users that have Xwayland installed system-wide and a different
Xwayland in their PATH still get their custom Xwayland.
- Users that don't have Xwayland installed system-wide but have it
somewhere else in PATH are left out. But this is pretty niche,
and they can just set WLR_XWAYLAND.
[1]: https://github.com/swaywm/wlroots/pull/2314
