treewide: Enforce C17 as the minimum required C standard

[carlescufi]

C99 has been the minimum required C standard version for Zephyr since its inception. After multiple attempts and discussions, a decision has been made to upgrade to C17 going forward.
This commits replaces the default C standard from C99 to C17 in the configuration and build system.

Closes #97061

[carlescufi]

@stephanosio @nashif @keith-packard should we also enforce C++17 at the same time, for consistency?

[pdgendt]

Do we need to drop these compiler feature flags? This will require all out-of-tree dependencies to drop this (even if they do support an older C standard version?).

I think this can remain, as newer standards should support older flags.

[tejlmand]

good catch.

Yes they should remain, cause they go into a list to tell CMake what standard features are available.
And there we should include old ones.

Some (external) code might do: target_compile_features(foo PRIVATE c_std_99) which is perfectly valid cause that just means code requires feature from c99, which is also supported by c17 (our new default).

[nvetrini]

@nashif You may want to consider switching to MISRA C:2012 Amendment 3 as a basis for the guidelines after updating the baseline C standard version, since that edition of the guidelines have much better coverage of C17 than Amendment 2 (e.g. it has guidelines for the safe use of _Generic, which are not present in Amendment 2)

[nashif]

DNM until we have verified all prequisites

[tejlmand]

please keep code in compiler_features.cmake.

The list should still include old standards as that will allow downstream users to request those as required (and it's support when using newer c17 std)

[kartben]

a decision has been made to upgrade to C17 going forward.

@carlescufi for tracking purposes, can you please link to the decision in question? I am not questioning that a decision has been made, but I am having a hard time tracking down when/where so I can only imagine it's even harder for mere mortals to find that information.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[carlescufi]

a decision has been made to upgrade to C17 going forward.

@carlescufi for tracking purposes, can you please link to the decision in question? I am not questioning that a decision has been made, but I am having a hard time tracking down when/where so I can only imagine it's even harder for mere mortals to find that information.

It's now linked to in the text of #97061