Don't use a builtin libcom_err if not using kerberos.

Builtin librarys like com_err should only be used if there is are proper
buildlink3.mk and builtin.mk files for the library, otherwise part of the
point of having the buildlink/builtin system is lost.

There no point in having a buildlink/builtin files for com_err as
currently only three packages use it by itself (mail/cyrus-imapd*) and
using the package suppled library only adds 72K to the package size.

(sbd)