Rationalize CI builds
We need to look at the CI and rationalize it a bit - we can't just keep adding more and more parallel builds. This is not a blocker for merging !303 (merged), but it's a blocker for much further development.
If we are aiming for Meson to become the recommended build system and a complete replacement for Autotools and CMake, then we can be fairly aggressive about pruning the Autotools and CMake build jobs: we can use Meson to fan out across different build configurations and (modern) OSs, and then cut down the Autotools and CMake builds that are run by default to maybe four (Autotools native, CMake native, Autotools mingw, CMake mingw - ideally with a mixture of Debian and openSUSE as host OS for widest coverage).