Similar to what was done for back-ndb, disable back-sql building when --enable-backends is given
https://git.openldap.org/openldap/openldap/-/merge_requests/71
Commits: • aa206be5 by Quanah Gibson-Mount at 2020-05-26T20:50:22+00:00 Issue#9234, Issue#9237 - drop back-sql, back-perl from --enable-backends