Clang 16 (c++20) build in CI fails

[elshize]

Clang 16 (c++20) build in CI fails:

https://github.com/pisa-engine/pisa/actions/runs/7865449374/job/21458386783

65.16 -- Compiling and running to test HAVE_POSIX_REGEX
65.20 -- Performing Test HAVE_POSIX_REGEX -- failed to compile