Merge pull request #2631 from ecoughlan/skip_invalid_setsockopt

Don't try to set IPV6_V6ONLY on OpenBSD

src/ip.cpp

@@ -108,7 +108,7 @@ void zmq::enable_ipv4_mapping (fd_t s_)
 {
   (void) s_;
 
-#ifdef IPV6_V6ONLY
+#if defined IPV6_V6ONLY && !defined ZMQ_HAVE_OPENBSD
 #ifdef ZMQ_HAVE_WINDOWS
     DWORD flag = 0;
 #else