only treat compilation warnings as error in CI

[ursfassler]

Summary

All warnings are treated as errors. This may lead to problems when using a different compiler version or during development.

Details

Now the warnings are only treated as errors in the CI.

Motivation and Context

Mentioned in #295.

How Has This Been Tested?

Manually by inspecting the actually used flags when Cmake is building the software. On the CI and locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• It is my own work, its copyright is implicitly assigned to the project and no substantial part of it has been copied from other sources (including Stack Overflow). In rare occasions this is acceptable, like in CMake modules where the original copyright information should be kept.
• I'm using the same code standards as the existing code (indentation, spacing, variable naming, ...).
• I've added tests for my code.
• I have verified whether my change requires changes to the documentation
• My change either requires no documentation change or I've updated the documentation accordingly.
• My branch has been rebased to main, keeping only relevant commits.