[WIP] fix: update MI_ATOMIC_VAR_INIT definition for C23 compatibility

[PastaPastaPasta]

Modified the definition of MI_ATOMIC_VAR_INIT to handle cases where ATOMIC_VAR_INIT is not defined or when compiling with C23, where it has been removed. This change ensures compatibility with newer C standards while maintaining support for older versions.

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[github-actions]

✅ No Merge Conflicts Detected

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[PastaPastaPasta]

@knst can you investigate if there's an issue with bleeding edge compilers?

[kwvg]

See bls-signatures#116 for alternative