gh-123681: Check NORMALIZE_CENTURY behavior at runtime; require C99

[encukou]

This is a continuation of #128444.

CPython requires C11, so all C99 features should work. We might need workaround if a supported platform misbehaves (hence this PR -- I want to feed the buildbots). But, IMO, any workarounds for unsupported platforms best left to the respective forks.

Serhiy's runtime check for normalize_century is left in; I don't know about the details there.

• Issue: gh-123681