If wither _ISOC23_SOURCE or --std=c23 (or higher), then default to the
making the ISOC visibility to 2023 or higher. This mirrors what glibc
does, so update the comment to specify that the _ISOCxx_SOURCE values.
We implement C11 and C23 versions (there's no C17 version, so we don't
invent that). We don't implement the C95, C99 or C2y versions. The
former two don't seem to be relevant, and the latter is tricky so defer
for now.
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