So I found this thread ([url]Seg fault while compiling ROOT6 in which one of the intermediate problems is identical (like, literally, it happens in the exact same place and everything) to what I have right now. The eventual solution there was to use MacPorts (I think?) after modifying/deleting the .rootrc file. I’m really really hesitant to touch MacPorts though, because it will break all of my Python libraries the second anything gets upgraded, and I’m not savvy enough to get them going again quickly. But in any case, I deleted the old .rootrc file that was leftover from v5, as well as all the files ever that were installed or built by v6, and attempted a clean build straight from the tarball.
It seems to have worked. Yay!