patch 7.4.1402
Problem:    GTK 3 is not supported.
Solution:   Add GTK 3 support. (Kazunobu Kuriyama)
diff --git a/src/eval.c b/src/eval.c
index 3b1172e..1471568 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -13671,7 +13671,11 @@
 #endif
 #ifdef FEAT_GUI_GTK
 	"gui_gtk",
+# ifdef USE_GTK3
+	"gui_gtk3",
+# else
 	"gui_gtk2",
+# endif
 #endif
 #ifdef FEAT_GUI_GNOME
 	"gui_gnome",