minor install bug with krb5 1.4 on NetBSD 1.6.2

[Greg Troxel]

I didn't give any thread options (which btw are not documented in the
install ps file in doc/), and configure failed finding pthread calls.
(NetBSD 1.6.2 has no threads, but one can install pth from pkgsrc.  I
had not done so.  NetBSD 2.0 has native POSIX threads.)

configure should detect that threading is just not available, and
build without thread support.

The release announcement says that some krb4 pieces are disabled by
default.  I was not able to figure out what those pieces are, and how
to enable them.  If you mean that by default the kdc only serves v5
and won't answer v4 requests unless you start it with an option to
enable v4, that's fine, but it might be good to reword things to say
that the disabling is a runtime, rather than compile-time option.  I
tried to figure out if there was a configure switch to reenable the
kdc answering v4 requests, but didn't find any.