I am a bit confused about the bdb and hdb backends. My understanding is that the former is based on the Berkeley database, whereas the latter uses an Oracle version of Berkeley database.
back-bdb and back-hdb are both built on top of the Berkeley DB database engine. They are both deprecated and will be removed from a future release. The supported and recommended backend for use is back-mdb. Oracle changed the license for BerkeleyDB such that 6.0 and later versions are not compatible with the OpenLDAP software. As for the hdb_open() perhaps the backends were compiled statically rather than dynamically.
Warm regards, Quanah -- Quanah Gibson-Mount Product Architect Symas Corporation Packaged, certified, and supported LDAP solutions powered by OpenLDAP: <http://www.symas.com>