in 3.x the _dbm extension can only be built with gdbm as the backend
library. gdbm is licensed under the GPL, so some people/projects like
Debian may have objections to build _dbm with the gdbm backend. It gets
difficult to determine, which code then uses _dbm/anydbm/... and then
check if it is compatible with the GPL.

Unless there is an alternative like dbm.sqlite (issue 3783), I would
like to re-add the detection of the BerkleyDB headers and libs, such
that _dbm can be built again without depending on gdbm. Adding a
configure flag to explicitely select one implementation (issue 4587)
would be helpful.

Ok to go ahead?