For compiling against a checkout of the stand-alone libmdb,
which has a branch 'mdb.master' with its own root:
    git clone --branch=mdb.master <openldap_repo_dir> mdb
    set_mdb="MDB_SUBDIR=`pwd`/mdb/libraries/libmdb"
    ... maybe modify mdb repository for testing ...
    cd <openldap_repo_dir>; make $set_mdb

Does not update the tests themselves.

In configure --help, call it "mdb database backend" instead of something
with "memory-mapped" which could confuse somebody about a memory-only DB.

Conflicts:

	servers/slapd/back-mdb/Makefile.in

Don't allow new pages for free list to come from the free list.
Otherwise a nasty data self-reference occurs that is too much
trouble to unwind.