Add an #ifdef guard around the use of IPV6_V6ONLY.

This definition is missing on Solaris 9. This was initially reported by
Dagobert Michelsen.
http://seclists.org/nmap-dev/2011/q4/15

CHANGELOG

@@ -1,5 +1,8 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o Fixed a compilation problem on Solaris 9 caused by a missing
+  definition of IPV6_V6ONLY. Reported by Dagobert Michelsen.
+
 o Upgraded included libpcap to version 1.2.1.
 
 o [NSE] Added hostmap-robtex.nse by Arturo Busleiman, which finds other

ncat/util.c

@@ -393,6 +393,7 @@ int do_listen(int type, int proto, const union sockaddr_u *srcaddr_u)
    http://msdn.microsoft.com/en-us/library/bb513665
 */
 #ifdef IPPROTO_IPV6
+#ifdef IPV6_V6ONLY
     if (srcaddr_u->storage.ss_family == AF_INET6) {
         int set = 1;
         /* Tell it to not try and bind to IPV4 */
@@ -400,6 +401,7 @@ int do_listen(int type, int proto, const union sockaddr_u *srcaddr_u)
             die("Unable to set IPV6 socket to bind only to IPV6");
     }
 #endif
+#endif
 
 #ifdef HAVE_SOCKADDR_SA_LEN
     sa_len = srcaddr_u->sockaddr.sa_len;