build: Don't run glib-mkenums if there's nothing to do

This fixes the build without libsoup.

Closes: #296
Approved by: gatispaeglis

src/libostree/ostree-dummy-enumtypes.c

@@ -1,7 +1,7 @@
 /* This file declares a stub function that is only exported
  * to pacify ABI checkers - no one could really have used it.
  *
- * Copyright (C) 2015 Red Hat, Inc.
+ * Copyright (C) 2016 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -23,7 +23,6 @@
 
 /* Exported for backwards compat - see 
  * https://bugzilla.gnome.org/show_bug.cgi?id=764131
- * https://github.com/ostreedev/ostree/pull/294
  */
 GType
 ostree_fetcher_config_flags_get_type (void)