Re: MacOS: xsltproc fails with "warning: failed to load external entity"

I wrote:
> It's worse than that: I find that
> export XML_CATALOG_FILES=/dev/null
> breaks the docs build on RHEL8 and Fedora 37 (latest) too, with the
> same "failed to load external entity" symptom. I conclude from this
> that there is no version of xsltproc anywhere that can still download
> the required files automatically. So we need to take out the advice
> that says you can rely on auto-download for everybody, not just macOS.

> If this is indeed the case, perhaps we ought to start inserting --nonet
> into the invocations. There's not much use in allowing these tools to
> perform internet access when the best-case scenario is that they fail.

Concretely, I'm thinking something like the attached. Notes:

1. I have not tested the meson changes.

2. As this is written, you can't override the --nonet options very
easily in the Makefile build (you could do so at runtime by setting
XSLTPROC, but not at configure time); and you can't override them at
all in the meson build. Given the lack of evidence that it's still
useful to allow net access, I'm untroubled by that. I did intentionally
skip using "override" in the Makefile, though, to allow that case.

3. For consistency with the directions for other platforms, I made
the package lists for macOS just mention libxslt. That should
be enough to pull in libxml2 as well.

4. Use of --nonet changes the error message you get if xsltproc
can't find the DTDs. I copied the error I get from MacPorts'
version of xsltproc, but can you confirm it's the same on Homebrew?

regards, tom lane