CLONE_NEWNS is only defined for __USE_GNU.

--- util/fusermount.c.orig	2011-06-27 17:41:08.845498441 +0200
+++ util/fusermount.c	2011-06-27 17:42:31.236498493 +0200
@@ -26,6 +26,7 @@
 #include <sys/fsuid.h>
 #include <sys/socket.h>
 #include <sys/utsname.h>
+#define __USE_GNU
 #include <sched.h>
 
 #define FUSE_COMMFD_ENV		"_FUSE_COMMFD"