bind: fix compilation when lmdb.h is present on host

bind: fix compilation when lmdb.h is present on host

Bind autoconf scripts look for lmdb.h in /usr/include (even when
cross-compiling). When liblmdb-dev is installed, this causes the
following error:

    ...
    checking for lmdb library... yes
    checking for library containing mdb_env_create... no
    configure: error: found lmdb include but not library.

Fix this by disabling explicitly lmdb support.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>
Signed-off-by: Julien Floret <julien.floret@6wind.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>