convert CI CompilerWarnings job to meson

This has been an open task in the move from autoconf to meson. It's
been discussed a few times here and there across various threads, but
it's gotten a bit confusing, so I'm starting a fresh thread.

The holdup was that the headerscheck target was not ported to meson.
This was done a few months ago. So converting the CI job to meson is
now somewhat straightforward. Here are some patches.

(I found some existing partial work in public git branches from Andres,
of which I have integrated a few bits here.)

I've tried to be careful to maintain the scope of each individual build
step (e.g., what features it enables, what optimization level it uses).

One thing we're losing is the sharing of the configure cache between the
steps (gcc.cache, clang.cache). There is a way to do that in meson
(meson-private/coredata.dat), but it's not meant to be used across
different build directories, and so we would have to restructure some of
this a bit differently, I think, so I stayed away from that for now.

The branch from Andres mentioned above was keen to enable PCH for the
mingw cross build for better performance. I have added that here as a
separate patch.

I've done a few rounds of testing and it all seems quite fast, but
obviously that depends on caching, time of day, and so on, so more
testing is welcome.