Fix build on SmartOS due to missing IPV6_V6ONLY.

Thanks to Matt Ingenthron.

Closes #1212.

ChangeLog.txt

@@ -47,6 +47,9 @@ Client features:
 - Add explicit support for TLS v1.3.
 - Drop support for TLS v1.0.
 
+Broker fixes:
+- Fix build on SmartOS due to missing IPV6_V6ONLY. Closes #1212.
+
 Client fixes:
 - mosquitto_pub wouldn't always publish all messages when using `-l` and
   QoS>0. This has been fixed.

src/net.c

@@ -470,8 +470,10 @@ int net__socket_listen(struct mosquitto__listener *listener)
 		ss_opt = 1;
 		setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &ss_opt, sizeof(ss_opt));
 #endif
+#ifdef IPV6_V6ONLY
 		ss_opt = 1;
 		setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, &ss_opt, sizeof(ss_opt));
+#endif
 
 		if(net__socket_nonblock(&sock)){
 			return 1;