Make sure config.h or the copy of that gets included only once to reduce duplicate include

warnings.

lib/driver/netbsd.c

@@ -1,5 +1,5 @@
 /*
-  Copyright (C) 2008, 2010 Rocky Bernstein <rocky@gnu.org>
+  Copyright (C) 2008, 2010, 2011 Rocky Bernstein <rocky@gnu.org>
 
   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
@@ -42,7 +42,8 @@
  */
 
 #ifdef HAVE_CONFIG_H
-#include "config.h"
+# include "config.h"
+# define __CDIO_CONFIG_H__ 1
 #endif
 
 #include <cdio/sector.h>