determining rdns capability

[Ken Dreyer]

For msktutil, I recently received a patch to optionally set "rdns =
false". In older versions of MIT (eg. on RHEL 5), this parameter has
no effect. I'd like to display a warning message if msktutil is built
against a version of MIT that does not support toggling rdns, so users
don't get confused.

What is the best way to determine MIT's rdns capability?

I initially thought I could check if KRB5_CONF_RDNS was defined, but
that header (src/include/k5-int.h) seems to be private.

- Ken