workaround problems with autoconf-2.63

http://lists.gnu.org/archive/html/autoconf/2009-04/msg00007.html

--- a/configure.ac
+++ b/configure.ac
@@ -3603,7 +3603,7 @@
 #include <shadow.h>
 	struct spwd sp;
 	],[ sp.sp_expire = sp.sp_lstchg = sp.sp_inact = 0; ],
-	[ sp_expire_available=yes ], []
+	[ sp_expire_available=yes ], [:]
 	)
 
 	if test "x$sp_expire_available" = "xyes" ; then