cross-build: Define GNUC_PREREQ in cdefs.h

This is required when including stdckdint.h and doesn't seem to be
provided by older clang.

Reviewed by: emaste
Fixes: 7233893e9496 ("lib{c,openbsd}: use ckd_mul() for overflow checking in re(c)allocarray")
Differential Revision: https://reviews.freebsd.org/D52933