Seems like you have a rather odd source checkout. I get no references to back-mdb at all. I do however find breakage in back-hdb I'm investigating.
That was my own fault (had 2.4.26 patches applied to RE24). Current RE24 builds fine for me, no references to back-mdb.
--Quanah -- Quanah Gibson-Mount Sr. Member of Technical Staff Zimbra, Inc A Division of VMware, Inc. -------------------- Zimbra :: the leader in open source messaging and collaboration