chg: dev: Detect and possibly define constexpr using Autoconf

Previously, we had an ISC_CONSTEXPR macro that was expanded to either
`constexpr` or `static const`, depending on compiler support.  To make
the code cleaner, move `constexpr` support detection to Autoconf; if
`constexpr` support is missing from the compiler, define `constexpr` as
`static const` in config.h.

Merge branch 'ondrej/rename-ISC_CONSTEXPR-to-constexpr-shim' into 'main'

See merge request isc-projects/bind9!9924