Compile fix for GTK 3.18: avoid gtk_adjustment_changed().

That function is deprecated as of 3.18, on the basis that GTK doesn't
need telling any more when the adjustment's owning widget needs
updating. So we just need to condition out the call.

unix/gtkwin.c

@@ -2833,7 +2833,9 @@ void set_sbar(void *frontend, int total, int start, int page)
     gtk_adjustment_set_step_increment(inst->sbar_adjust, 1);
     gtk_adjustment_set_page_increment(inst->sbar_adjust, page/2);
     inst->ignore_sbar = TRUE;
+#if !GTK_CHECK_VERSION(3,18,0)
     gtk_adjustment_changed(inst->sbar_adjust);
+#endif
     inst->ignore_sbar = FALSE;
 }