[libwnck] utils: add a warning if no X11 is found

[Marco Trevisan <marcotrevi src gnome org>]

commit c4cc2eb15f9da7a02ff584d5f517239b53a2c985
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Mon Nov 28 14:52:37 2016 +0100

    utils: add a warning if no X11 is found

 libwnck/util.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/libwnck/util.c b/libwnck/util.c
index bc235bb..c9a75ba 100644
--- a/libwnck/util.c
+++ b/libwnck/util.c
@@ -812,7 +812,11 @@ _wnck_get_default_display (void)
   /* FIXME: when we fix libwnck to not use the GDK default display, we will
    * need to fix wnckprop accordingly. */
   if (!GDK_IS_X11_DISPLAY (display))
-    return NULL;
+    {
+      g_warning ("libwnck is designed to work in X11 only, no valid display found");
+      return NULL;
+    }
+
   return GDK_DISPLAY_XDISPLAY (display);
 }