Without this patch, NULL is undefined with glibc 2.14.1.

--- ./libicmp/icmp_timestamp.c.bad	2007-06-29 00:58:23 +0200
+++ ./libicmp/icmp_timestamp.c	2011-10-26 16:25:04 +0200
@@ -21,6 +21,7 @@
 # include <config.h>
 #endif
 
+#include <stdlib.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/time.h>