Replace call_once with an atomic exchange to log time constant warnings.

[nkoukpaizan]

Description

This replaces std::once_flag/std::call_once with std::atomic<bool>/ .exchange(true) to log time constant warnings. This should be more portable (including on older linux systems).

Closes #567

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• [N/A] There are unit tests for the new code.
• [N/A] The new code is documented.
• The feature branch is rebased with respect to the target branch.
• [N/A] The CHANGELOG.md has been updated to reflect the changes. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments

[Steven-Roberts]

This fixes the build issue I had earlier.