Wt::Dbo is a self-contained library which implements an Object-Relational Mapper (ORM) for C++, and thus a convenient way to interact with SQL databases from C++.
Although features like optimistic concurrency control make this an ideal technology for a database driven web applications (and it provides good integration with Wt's MVC classes), the library can also be used for other applications, and does not depend on Wt.
See also this tutorial for an idea of what Dbo looks like.
The library abstracts different deployment options as connector libraries, which connect Wt with the outer world. Switching deployment option is a matter of (re)linking to one of these connector libraries!
When in doubt, pick the built-in httpd. It's convenient, battle-tested, and supports the most features.