Support building against OpenLDAP 2.6+
    
    OpenLDAP 2.6 deprecated separate libldap/libldap_r, there is only one
    (reentrant) variant for the library.
    
    Attempt to use _r variant by default. In case it is missing, assume we
    are using OpenLDAP 2.6 which has libraries without _r suffix. The
    functions are still reentrant so there is not functional difference.
    
    Fixes: https://pagure.io/freeipa/issue/9080
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>