Fix configure help message for Berkeley DB 6.
Addon to r1495887 (trunk) resp. r1528827 (1.5.x).

Backport of r1495889 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr-util/branches/1.5.x@1528839 13f79535-47bb-0310-9956-ffa450edef68