window: GtkPlug is conditionally supported on X11 only

Apply an upstream patch to fix a build regression in 3.14.5. This patch
will be included in the next 3.14.6 release, so we can remove it in the 
next version bump.

Error message:

./.libs/libgtk-3.so: undefined reference to `GTK_IS_PLUG'

Bug report:
  https://bugzilla.gnome.org/show_bug.cgi?id=739885

Upstream commit:
  https://git.gnome.org/browse/gtk+/commit/?id=36fedb0c4b373272afc2d76a7c790d696ed48034

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>

From 36fedb0c4b373272afc2d76a7c790d696ed48034 Mon Sep 17 00:00:00 2001
From: Emmanuele Bassi <ebassi@gnome.org>
Date: Mon, 10 Nov 2014 14:08:20 +0000
Subject: [PATCH] window: GtkPlug is conditionally supported on X11 only

We cannot do a type check on it if we don't have the X11 backend
compiled in.

https://bugzilla.gnome.org/show_bug.cgi?id=739885
---
 gtk/gtkwindow.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/gtk/gtkwindow.c b/gtk/gtkwindow.c
index 0d0fb88..6cb355b 100644
--- a/gtk/gtkwindow.c
+++ b/gtk/gtkwindow.c
@@ -1610,10 +1610,17 @@ gtk_window_constructed (GObject *object)
 {
   GtkWindow *window = GTK_WINDOW (object);
   GtkWindowPrivate *priv = window->priv;
+  gboolean is_plug;
 
   G_OBJECT_CLASS (gtk_window_parent_class)->constructed (object);
 
-  if (priv->type == GTK_WINDOW_TOPLEVEL && !GTK_IS_PLUG (window))
+#ifdef GDK_WINDOWING_X11
+  is_plug = GTK_IS_PLUG (window);
+#else
+  is_plug = FALSE;
+#endif
+
+  if (priv->type == GTK_WINDOW_TOPLEVEL && !is_plug)
     {
       priv->multipress_gesture = gtk_gesture_multi_press_new (GTK_WIDGET (object));
       gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (priv->multipress_gesture), 0);
-- 
1.7.1