Bump minimum GTK for PGTK builds

* configure.ac: Require GTK 3.20 or later if building with PGTK.
It has already been required for some time now, but the
requirement was not reflected in the configure script.

* src/pgtkterm.c (pgtk_any_window_to_frame): Remove version
check.

configure.ac

@@ -2797,7 +2797,11 @@ gtk3_pkg_errors=
 if test "${opsys}" != "mingw32"; then
   if test "${with_gtk3}" = "yes" || test "${with_gtk}" = "yes" || test "$USE_X_TOOLKIT" = "maybe"; then
     GLIB_REQUIRED=2.37.5
-    GTK_REQUIRED=3.10
+    if test "${window_system}" = "x"; then
+      GTK_REQUIRED=3.10
+    else
+      GTK_REQUIRED=3.20
+    fi
     GTK_MODULES="gtk+-3.0 >= $GTK_REQUIRED glib-2.0 >= $GLIB_REQUIRED"
 
     dnl Checks for libraries.

src/pgtkterm.c

@@ -5008,7 +5008,6 @@ pgtk_any_window_to_frame (GdkWindow * window)
 static gboolean
 pgtk_handle_event (GtkWidget *widget, GdkEvent *event, gpointer *data)
 {
-#if GTK_CHECK_VERSION (3, 18, 0)
   struct frame *f;
   union buffered_input_event inev;
   GtkWidget *frame_widget;
@@ -5045,7 +5044,6 @@ pgtk_handle_event (GtkWidget *widget, GdkEvent *event, gpointer *data)
 
       return TRUE;
     }
-#endif
   return FALSE;
 }