According to the web page description of Courier-IMAP, virtual mailboxes uses GDBM or DB database to translate arbitrary users to maildirs. But if I won't be doing that, do I need GDBM or a DB database at all for anything else? If not, can it be left out of the ./configure step?

I won't be using virtual mailboxes in the sense of the old style unix way of translating "some...@example.com" to local user "somebody". Instead, all mail will be delivered into a distinctive mailbox structured like:

/var/mail/example.com/someone/maildir

With a direct construction of the path, it's not really virtual anyway. So the translation is not needed, either. Delivery and pickup both need to do the same construction, but that should be quite simple to set up (maybe a small hack if not already implemented).