modules: Don't use GDK_THREADS_ENTER/LEAVE macros internally

These are just wrappers for the functions, and we want to
deprecate them. Stopping to use them internally is a good
first step. Also define GTK_COMPILATION so we can keep using
gdk_threads_enter/leave without causing deprecation warnings.

modules/printbackends/file/gtkprintbackendfile.c

@@ -401,11 +401,11 @@ file_print_cb (GtkPrintBackendFile *print_backend,
                GError              *error,
                gpointer            user_data)
 {
-  GDK_THREADS_ENTER ();
+  gdk_threads_enter ();
 
   file_print_cb_locked (print_backend, error, user_data);
 
-  GDK_THREADS_LEAVE ();
+  gdk_threads_leave ();
 }
 
 static gboolean