Casual Meson fixups

The first two patches in the series are re-proposals that had previously
been approved[0] by Andres, but fell through the cracks.

The only patch that _could_ be controversial is probably the last one,
but from my understanding it would match up with the autotools build.

One thing that I did notice while testing this patch is that Muon
doesn't build postgres without coercing the build a bit. I had to
disable nls and plpython. The nls issue could be fixed with a bump to
Meson 0.59, which introduces import(required:). nls isn't supported in
Muon unfortunately at the moment. The plpython issue is that it doesn't
understand pymod.find_installation(required:), which is a bug in Muon.

Muon development has slowed quite a bit this year. Postgres is probably
the largest project which tries its best to support Muon. It seems like
if we want to keep supporting Muon, we should get a buildfarm machine to
use it instead of Meson to catch regressions. OR we should contemplate
removing support for it.

Alternatively someone (me?) could step up and provide some patches to
Muon to make the postgres experience better. But I wonder if any
Postgres user even uses Muon to build it.

--
Tristan Partin
Neon (https://neon.tech)