These constants are GNU libc extensions that are likely to be adopted
by the next POSIX revision [1]. The definitions can be verified in
a PARI-GP shell session:
Put these behind __BSD_VISIBLE || __XSI_VISIBLE >= 800 to future-proof
these changes. They shouldn't be defined at lower levels of XSI, but don't
have other XSI 800 stuff in place yet.