At http://documentation.magnolia-cms.com/modules/ldap.html#Installing

There is info border which says

Unlike most modules, installing the LDAP module doesn't trigger upgrade or installation tasks. It will quietly make the necessary classes available for your configuration, but you won't see it listed in the modules section of your configuration, nor will it trigger the "Upgrade Needed" notification.

but from ldap version 1.6.1 ldap modules is real magnolia module. So you can see it listed in the modules section of your configuration and also it will trigger the "Upgrade Needed" notification.

There is also

Add a new property jndi.ldap.config in the magnolia.properties file and set its value to a relative path to the webapp.

but it can also be an absolute path, or ${magnolia.home} properties can be used.