exim + gdbm with FreeBSD?

I'm trying to get exim 1.61 to use gdbm on a FreeBSD box, without much luck.

I've changed the makefile and linked in gdbm (-g), and it seems to me that
it's doing its own locking and so exim can't ever open wait-smtp or retry
for writing, even when newly-created. Have I missed something here?

FWIW, I'm doing this because I suspect a bug in the db package. I'm getting a
heap of dbm-related problems whilst subjecting exim to intense load, including
trashed .db files, memory hogging, SEGVs and looping deep inside db->get/put.

Ian.