Replace gtk_debug_flags with getter and setter functions

Preferrably should be made just into a local variable for libgtk like
_gdk_debug_flags for libgdk. But for now used by
gtk/tests/textbuffer.c and modules/printbackends/cups/gtkprintbackendcups.c.

modules/printbackends/cups/gtkprintbackendcups.c

@@ -2342,7 +2342,7 @@ cups_request_ppd (GtkPrinter *printer)
 
 #ifdef G_ENABLE_DEBUG 
   /* If we are debugging printing don't delete the tmp files */
-  if (!(gtk_debug_flags & GTK_DEBUG_PRINTING))
+  if (!(gtk_get_debug_flags () & GTK_DEBUG_PRINTING))
     unlink (ppd_filename);
 #else
   unlink (ppd_filename);