I picked it up from:
http://www.rigsofrods.com/wiki/images/c/c0/Socketw.patch

--- a/src/sw_base.cxx.old	2009-12-19 21:19:31.057051328 -0800
+++ b/src/sw_base.cxx	2009-12-19 21:19:44.939551918 -0800
@@ -19,6 +19,7 @@
 #include <time.h>
 #include <stdio.h>
 #include <string.h>
+#include <stdlib.h>
 
 #ifndef __WIN32__
   #include <netdb.h>