patch 8.1.0368: GTK code has too many #ifdefs and GTK 2.10 building fails

Problem:    GTK code has too many #ifdefs and building fails with GTK 2.10.
Solution:   Always use gtk_widget_get_window() and define it for older GTK
            versions. (Ken Takata, closes #3421)
diff --git a/src/vim.h b/src/vim.h
index ece5414..6d1c312 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1241,7 +1241,7 @@
 #define MIN_SWAP_PAGE_SIZE 1048
 #define MAX_SWAP_PAGE_SIZE 50000
 
-/* Special values for current_SID. */
+/* Special values for current_sctx.sc_sid. */
 #define SID_MODELINE	-1	/* when using a modeline */
 #define SID_CMDARG	-2	/* for "--cmd" argument */
 #define SID_CARG	-3	/* for "-c" argument */
@@ -2329,6 +2329,12 @@
 # endif
 #endif
 
+#ifdef FEAT_GUI_GTK
+# if !GTK_CHECK_VERSION(2,14,0)
+#  define gtk_widget_get_window(wid)	((wid)->window)
+# endif
+#endif
+
 #ifndef FEAT_NETBEANS_INTG
 # undef NBDEBUG
 #endif