Makefile: Hook MIT KRB5 into the build

Add tests for MK_MITKRB5. If "yes" build MIT KRB5. If "no" build Heimdal.
The default is MK_MITKRB5 = no, added by "krb5: Add build plumbing".

At some point we will change the default to MK_MITKRB5 = yes. A ports
exp-run will need to be successfully run first.

Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D50815