make -Wall, -Wextra and /W4 private to the project

[daantimmer]

This PR sets the -Wall, -Wextra and /W4 flags scoped to just the libcoro project.
It also removes the duplicate flags in libcoro_test and removed flags -fcoroutines and -fconcepts (these are set by default when the language version is set to C++20)

• ref: https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#index-fcoroutines
• ref: https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#index-fconcepts

-std=c++20 could be removed from libcoro_test because its always set through CMAKE_CXX_STANDARD
-fexceptions is removed because its set publicly in libcoro

Fixes #453

[daantimmer]

So, I can also confirm from a 'consuming project' point of view that this is now properly set 'privately':

currently using my forked version here: philips-software/amp-cucumber-cpp-runner#312

And the actual commit with that change (and what I could remove because of it): philips-software/amp-cucumber-cpp-runner@963455f

[jbaldwin]

I think the windows build is busted because GitHub added windows-2025 which windows-latest now points at. I think if you adjust the tank to be windows-2022 it should work again and we can get a new issue to get 2025 working.

[daantimmer]

Give me a sec to fix that windows issue. Apparently the CMAKE_CXX_COMPILER_ID isn't set properly for MSVC, although it should. It was the misuse of CMAKE_CXX_COMPIELR_ID, we shouldn't use ${CMAKE_CXX_COMPILER_ID} MATCHES "..." but use CMAKE_CXX_COMPILER_ID MATCHES "..." instead. Without the ${} as demonstrated here: https://godbolt.org/z/ThcbG9zdo Don't pay attention to the missing output of MSVC, it tries to read .exe.exe

[daantimmer]

@jbaldwin its ready for review btw, no more changes required from my side :)