dc4c0397eb
The contexpr introduced in C23 standard makes perfect sense to be used
instead of preprocessor macros - the symbols are kept, etc. Define
ISC_CONSTEXPR to be `constexpr` for C23 and `static const` for the older
C standards. Use the newly introduced macro for the NS_PER_SEC and
friends time constants.
(cherry picked from commit 122a142241)