[Samba] Need to upgrade very old samba: 4.6 AD to current release

[xyz]

Hello
I need to upgrade Samba v.4.6 with AD enabled, which is built from sources
and installed on Ubuntu 12.04.

Trying .configure script in v.4.9 it complains: Samba AD DC and
--enable-selftest requires lmdb 0.9.16 or later

In this state of the outdated system upgrade will fail due to missed
dependencies.

I planned to upgrade OS before parallel to samba upgrade, but sadly repos
of Ubuntu 12.04 and 14.04 has been removed.

Maybe I should install ubuntu 16.04 on the test machine, restore the backup
of the samba installation on it and try subsequent updates? Or should I
install a second 4.6 domain controller on a newer OS, transfer roles, and
try more updates

Any other options giving the chance for success this update?