back-mdb/Makefile.in += variable MDB_SUBDIR.

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