Running "make check" while offline or on a firewalled network may
result in failure in lib/krb5/os because the invocation of
t_locate_kdc requires that the DNS servers for ATHENA.MIT.EDU be
reachable. Autodetect DNS utilities "dig" and "nslookup", and use
them to check for existence of the known-good DNS name. Also
parameterize the test so that the known-good DNS name can be
overridden on the make command line.
Commit By: tlyu