request: provide a canonical autoconf fragment for finding krb5 and/or gssapi

[Sam Hartman]

I propose we treat both Derrick and Love's requests as 1.4.1 issues on
the MIT side provided that we don't end up blocking on any Heimdal
coordination issues.

I don't think we can remove runtime path arguments from ldflags now
though; that might break existing applications.  We may need a new
argument.

--Sam