pthread: compiling ntp_dns needs -pthread, linking needs -pthread

The man page for pthread_create says:

   Compile and link with -pthread.

Note, no l after the -

Argh. It's more complicated than that. The above is from Linux.

NetBSD and FreeBSD say:

 POSIX Threads Library (libpthread, -lpthread)

---

ntp_dns and the pthread library can be omitted if configured with --disable_dns_lookup The crypto stuff uses pthread_once() so it may drag in libpthread anyway.