Upstream: http://sourceforge.net/mailarchive/forum.php?thread_name=4CB2549C.2030100%40gmail.com&forum_name=prboom-devel

--- prboom-2.5.0/src/SDL/i_system.c.orig	2010-10-11 09:58:02.000000000 +1030
+++ prboom-2.5.0/src/SDL/i_system.c	2010-10-11 09:59:16.000000000 +1030
@@ -33,6 +33,7 @@
  *-----------------------------------------------------------------------------
  */
 
+#define _GNU_SOURCE
 #include <stdio.h>
 
 #include <stdarg.h>