It turns out my analysis was a bit premature. Yes, when trying to "make test" it segfaults, and yes,
this seems to depend on the sqlite3 package, but no, it is not the sqlite3 3.9.2 package that was
built that segfaults; what segfaults is a sqlite3 3.16.0 package Apple bundled with the OS.
If forcing a load of 3.9.2 it works fine (AFAICT; I haven't figured out how to make the tests do that,
just ran tdbc::sqlite3::connection create and some other commands manually in a Tcl shell,
not seeing anything wrong).

There still is a bug, namely that "make test" isn't testing the package we built but some random
higher version-numbered package it happens to find already installed, however that probably
isn't a critical error.

There is also a gotcha, that this OS-provided package (the ifneeded script reads
load Tclsqlite3) is out there and disrupts things,
but that's not something the TDBC maintainers can do anything about directly. (Although
updating the bundled sqlite3 version should help.)