Add LDAPS capability to RT::Authen::ExternalAuth

I've added the capability to use Net::LDAPS instead of vanilla Net::LDAP, and provided a single configuration option to switch back and forth, which is reflected in the example configuration.

I've also modified the Makefile to check for a working Net::LDAPS.

The pre-existing codebase only allows for the use of TLS. A full discussion of the difference can be found here:
http://search.cpan.org/~gbarr/perl-ldap-0.43/lib/Net/LDAP/Security.pod#How_does_an_LDAPS_connection_work