Fix the socklen_t type too. Needed only when built with memory debugging

as then we include the memdebug.h header from the lib dir, and it then
requires this type... (fails on IRIX 6.5 without this)

src/config.h.in

@@ -89,3 +89,5 @@
 /* Define if you have the <limits.h> header file */
 #undef HAVE_LIMITS_H
 
+/* type to use in place of socklen_t if not defined */
+#undef socklen_t