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