only try gtk3 vclplug on >= 3.14

themeing has changed around quite a bit, and there's insufficient interest to
get older versions looking right

Change-Id: Ia2ee58d66c859e3bf00b54413fac1efb00d38dd3
(cherry picked from commit f82e6424)

vcl/unx/gtk/app/gtkinst.cxx

@@ -82,6 +82,12 @@ extern "C"
             XInitThreads();
 
 #if GTK_CHECK_VERSION(3,0,0)
+        if (gtk_minor_version < 14)
+        {
+            g_warning("require a newer gtk than 3.%d for theme expectations", gtk_minor_version);
+            return NULL;
+        }
+
         const gchar* pVersion = gtk_check_version( 3, 2, 0 );
 #else
         const gchar* pVersion = gtk_check_version( 2, 2, 0 );