xwayland: check executable exists on init

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

include/meson.build

@@ -6,6 +6,8 @@ if not features.get('x11-backend')
 endif
 if not features.get('xwayland')
 	exclude_files += 'xwayland.h'
+else
+	subdir('xwayland')
 endif
 if not features.get('xdg-foreign')
 	exclude_files += [