Re: Headerscheck support for meson

Hello

See attached v2, which uses a meson-integrated version while only
duplicating some of the code (and adding some python scripts).

It uses static libraries, at first I tried to exclude it from the
default build and add it as a test dependency, so it only gets built
as part of `meson test` (but only once).

I had multiple problems with this:

* Meson before 1.7 ignores this and builds it anyway
* Doing it in 2 steps is significantly slower than everything
together, I guess it's better if the minimal work and real work
compilations get mixed together.

On my laptop, meson build time went from ~7 seconds to ~10 seconds
with headerscheck enabled, and I didn't see any difference in
configuration time. I also added a configuration flag for it in case
somebody wants to disable it, or if it's slower in other environments,
but it is enabled by default.

The header-guessing part is duplicated instead of using a common
configuration file as that seemed smaller overall. The python code is
a bit uglier because it uses chained ifs - I'm not sure what's the
minimum python version the postgres builds officially requires? The
current meson version depends only on 3.6, so I didn't want to use
pattern matching.

Compared to headerscheck, this build can't use -fsyntax-only because
meson needs build artifacts - but those are basically empty, so it
shouldn't matter that much. And thanks to ninja, it's parallel!

What do you think?