Support finding Kerberos via pkg-config

[list at johnericson.me]

Hello, I proposed these changes a few months back in
https://github.com/openssh/openssh-portable/pull/219, but am only seeing
now patches are meant to be submitted via the mailing list rather than
GitHub. Sorry for using the wrong mechanism.

As the P.R. says, the main motivation is that it makes cross compilation
easier. But I might submit that supporting pkg-config where possible is
good practice in general.

Thanks,

John