Check existence of IPV6_V6ONLY in evutil_make_listen_socket_ipv6only(…

…) (mingw32)

MinGW 32-bit 5.3.0 does not defines it and our appveyour [1] build
reports this instantly:
    evutil.c: In function 'evutil_make_listen_socket_ipv6only':
    evutil.c:392:40: error: 'IPV6_V6ONLY' undeclared (first use in this function)
      return setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, (void*) &one,

  [1]: https://www.appveyor.com/docs/windows-images-software/#mingw-msys-cygwin

Another solution will be to use mingw64 which has it, but I guess we do
want that #ifdef anyway.

evutil.c

@@ -388,9 +388,12 @@ evutil_make_listen_socket_reuseable_port(evutil_socket_t sock)
 int
 evutil_make_listen_socket_ipv6only(evutil_socket_t sock)
 {
+#if defined(IPV6_V6ONLY)
 	int one = 1;
 	return setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, (void*) &one,
 	    (ev_socklen_t)sizeof(one));
+#endif
+	return 0;
 }
 
 int

include/event2/util.h

@@ -451,7 +451,8 @@ int evutil_make_socket_closeonexec(evutil_socket_t sock);
     socket() or accept().
 
     @param sock The socket to be closed
-    @return 0 on success, -1 on failure
+    @return 0 on success (whether the operation is supported or not),
+            -1 on failure
  */
 EVENT2_EXPORT_SYMBOL
 int evutil_closesocket(evutil_socket_t sock);