Reapply CPP20 compatibility and enable check in CI

[ThoFrank]

Changes from previous PR #8 were overwritten. This reapplies them and enables CI builds for cpp 14,17 and 20.

[ThoFrank]

@mthiede-acn2 could you please take a look? The changes from #8 are all done in one commit, but I had to patch one more file.

[marcmo]

this will of course extend our build times. Would probably be enough to just run the compiler front end without generating code. We can use the compiler’s syntax-checking mode or run only the front-end to parse by using the -fsyntax-only flag.

[ThoFrank]

I don't think -fsyntax-only is supported by CMake. It would probably require a bigger hack. I tried it naively by adding -DCMAKE_C_FLAGS which resulted in CMake complaining about a broken compiler which doesn't produce binaries.

We could add ccache in the future. This could help bring down compilation times. Somebody already created a ccache-action which combines ccache with github's cache.

[marcmo]

Regarding the C++ 20 compatibility changes, some of them seem to be highly controversial.
why is removing volatile valid in all those places? we cannot just remove it!
getting rid of volatile AND the compound assignment for sure gets rid of the warnings, but it changes the code.

and..does it really make sense to get rid of compound assignments on volatile anyway?
I'd rather NOT add C++ 20 support as long as we don't have the need...and then only do it after careful consideration. @matthiaskessler ?

[marcmo]

do not just remove volatile

[ThoFrank]

I just reapplied the changes from #8. They were already approved and merged before the migration to GitHub.
So I didn't think it would be controversial. I combined all the changes from #8 into one commit. My changes are done in other commits