update for mingw's default _WIN32_WINNT

mingw defaults with _WIN32_WINNT as 0x0502 which doesn't define inet_pton(), so add a conditional check

tests/test_heartbeats.cpp

@@ -185,7 +185,11 @@ test_heartbeat_timeout (void)
 
     ip4addr.sin_family = AF_INET;
     ip4addr.sin_port = htons(5556);
-    inet_pton(AF_INET, "127.0.0.1", &ip4addr.sin_addr);
++#if (ZMQ_HAVE_WINDOWS and _WIN32_WINNT < 0x0600)
++    ip4addr.sin_addr.s_addr = inet_addr ("127.0.0.1");
++#else
++    inet_pton(AF_INET, "127.0.0.1", &ip4addr.sin_addr);
++#endif
 
     s = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
     rc = connect (s, (struct sockaddr*) &ip4addr, sizeof ip4addr);