Aha, I was able to run a "make -i install-world", which ignored the SGML errors and built the rest of the world. Now trying "make -i installcheck-world".
So I ran "make -i installcheck-world" and it ran to completion. This is on a freshly "git pull"ed postgres source tree. Certain tests failed, but most succeeded. There was no "overall" indication of success or failure at the very end, so I'm not sure what this means.
The only "unusual" thing I'm doing that I'm aware of is I'm setting the installation directory to "/path/to/git/of/postgres/install_dir" rather than wherever it installs by default. I've attached my configure options and the result of the "make -i installcheck-world", in the hopes that someone can tell me whether this is an acceptable result or if it means there's something wrong with my install? I'd like to be able to review patches, and I'm not sure if this result I just got indicates a success or a failure of "make installcheck-world".
Please let me know if there's any more info I should provide.