How to access "vec_avg" / "vec_mean"?

[thjbdvlt]

I'm discovering this very cool project and really like how the documentation is written (its style), even if it's not complete atm.
The documentation mentions a vec_mean function, and I find a few occurrence of vec_avg in the code, but I can't call any from SQLite (neither can I call other aggregate function such as vec_min).
Do I need to enable some build flags? Or are they not yet ready (which would not be an issue, since I just have to get the data and do the average by myself)?
I've seen that the project is not actively maintained, so I could help to add stuff to doc, probably not to the code as I don't feel very capable for such field (vectors).