Enable NTLM only if code is compiled against Heimdal

Make NTLM only work if we are on Heimdal GSS-API implementation which is most true for FreeBSD, etc. otherwise advertise SPNEGO only.
Thanks for the hint. I am not a day-to-day C hacker. What I can do is to externalize the check to an `int` with `#ifdef` and pass that to the if clase. Does that make sense?