Check existence of IPV6_V6ONLY in evutil_make_listen_socket_ipv6only() (mingw32)

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.