The mdb backend was introduced in openldap 2.4.27.
Please add a use flag to be able to use the mdb backend.
Created attachment 364404 [details]
openldap-2.4.35-r1.ebuild with an mdb use flag to pass --enable-mdb or --disable-mdb to ./configure.
However, please note the following from "man 5 slapd-mdb":
This is an early release; the database file format or other characteristics may change incompatibly in future releases.
Hence it might instead be a better idea to enable mdb only with USE="experimental" instead.
Hmm, it's enabled by default in the configure script, unless you explicitly disable it. So unless you want a flag to be able to *disable* it, what you've got should be working for you... I'm running openldap with the current ebuild using mdb.
Btw, upstream considers mdb stable enough for production use in current openldap versions.