configure.in patch for krb support

[David W. Chapman Jr.]

> > In 3.0 I noticed that users reported build breakages in krb, but they
> > didn't use krb.  This patch was sent to me my petef at freebsd.org,
> > would someone review it and possibly commit if its applicable?
>
> While kerberos used to be an ugly little bit tacked on the side of
> Samba, it now is used for AD functionality.  As such we want to use it
> as much as possible.
>
> If there are problems with kerberos on a particular system, they should
> be fixed by making the configure test work properly :-).
>

It currently is broken on FreeBSD, what information would be helpful in
debugging the problem and finding out whether its samba to blame or FreeBSD?