> I guess the question is whether all the code is third-party or simply
> optional on some OS? Don't know the answer off the top of my head.
In my Debian world it’s typical to use only the official repos, there are no third parties (except from the viewpoint of Python).  I used “system” instead of “third-party” in my proposition.  For me on Debian the optional deps really are “not installed at Python build time” (“not installed by default” or “optional” doesn’t really make sense, as there are multiple defaults, and 98% of the packages are not required to have a working system).

> I personally don't like the message re-ordering. It feels like "oops, you
> didn't build everything. Hey everything built fine!"
Well I distinguished “interpreter” and “optional modules” on purpose, not “everything”, but I agree it can be a fine distinction when you’re just getting started.  Let’s not reorder the messages.

> And it isn't necessarily system files. I mean sqlite3 is not a system package
Really?  If it doesn’t build because sqlite3 header files are not found, then that’s a missing system file to me.

(I used “file” instead of “package” because not everybody will install deps using the system package manager (if any), but that may be complicating things for no good reason.)