freebsd9: support both WAF MIT krb5 build and autoconf build against MIT krb5

freebsd9: support both WAF MIT krb5 build and autoconf build against MIT krb5

System-provided Heimdal Kerberos in FreeBSD 9 lacks proper support for parsing MS PAC.
This leaves us with MIT krb5 package from ports or embedded Heimdal in source4.
MIT krb5 from ports is 1.9.2, it supports all needed features for AD support in smbd,
as well as WAF MIT krb5 build. In order to use it, one needs to install 'krb5' package.

Autoconf build:
  --with-krb5=/usr/local

WAF build:
  --with-system-mitkrb5 /usr/local

or otherwise krb5-config from system Heimdal will overtake and break the detection, leaving
you with a mixture of Kerberos libraries from different locations.

WAF build accepts multiple paths as sub-arguments of the --with-system-mitkrb5 and searches
through them for krb5-config, i.e. /usr/local /usr/kerberos ...

Autobuild-User: Alexander Bokovoy <ab@samba.org>
Autobuild-Date: Mon May 28 23:40:30 CEST 2012 on sn-devel-104