doc-reference.py pointing reference base dir wrongly at DESTDIR
As used by glibmm meson build, doc-reference.py ends up building the devhelp file so that <book base="...">
points at DESTDIR, which is just a temporarily location for the build. This seems to end up having the devhelp book completely broken after installation, with devhelp saying the file from DESTDIR can't be (obviously) found.
That said, after hacking around that, it's still resulting in non-working docs in devhelp, so I'm not sure what else is going on. But that seems to be a problem shared with glibmm autotools builds as well.
But this part at least is different and results in apparently definitely not working devhelp book, so kicking off some discussion and fixing things based on that, unless I'm doing something very wrong here.