Update configure's probe for libldap to work with OpenLDAP 2.5.

Update configure's probe for libldap to work with OpenLDAP 2.5.

The separate libldap_r is gone and libldap itself is now always
thread-safe.  Unfortunately there seems no easy way to tell by
inspection whether libldap is thread-safe, so we have to take
it on faith that libldap is thread-safe if there's no libldap_r.
That should be okay, as it appears that libldap_r was a standard
part of the installation going back at least 20 years.

Report and patch by Adrian Ho.  Back-patch to all supported
branches, since people might try to build any of them with
a newer OpenLDAP.

Discussion: https://postgr.es/m/17083-a19190d9591946a7@postgresql.org