problem: DragonFly BSD does not allow mapping global IPv4 addresses when IPv6 is used

solution: Handle as for OpenBSD by not attemption to set this option on the socket
Signed-off-by: Christopher Hall <hsw@ms2.hinet.net>

src/ip.cpp

@@ -129,7 +129,7 @@ void zmq::enable_ipv4_mapping (fd_t s_)
 {
     LIBZMQ_UNUSED (s_);
 
-#if defined IPV6_V6ONLY && !defined ZMQ_HAVE_OPENBSD
+#if defined IPV6_V6ONLY && !defined ZMQ_HAVE_OPENBSD && !defined ZMQ_HAVE_DRAGONFLY
 #ifdef ZMQ_HAVE_WINDOWS
     DWORD flag = 0;
 #else