Bug 1833855 [Linux] Make sure we have correct display for X11 and Wayland exclusive builds r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D178639

toolkit/xre/nsAppRunner.cpp

@@ -4745,6 +4745,18 @@ int XREMain::XRE_mainStartup(bool* aExitFlag) {
     else {
       gdk_display_manager_open_display(gdk_display_manager_get(), nullptr);
     }
+#  endif
+    // Check that Wayland only and X11 only builds
+    // use appropriate displays.
+#  if defined(MOZ_WAYLAND) && !defined(MOZ_X11)
+    if (!GdkIsWaylandDisplay()) {
+      Output(true, "Wayland only build is missig Wayland display!\n");
+    }
+#  endif
+#  if !defined(MOZ_WAYLAND) && defined(MOZ_X11)
+    if (!GdkIsX11Display()) {
+      Output(true, "X11 only build is missig X11 display!\n");
+    }
 #  endif
   }
 #endif