Code introduced in #15649 added usage of `timingsafe_bcmp()`, if
available, otherwise falling back to our own implementation. However
the relevant build system check was never added, so currently, we'll
always just use our implementation, as HAVE_TIMINGSAFE_BCMP will never
be defined.
Add the check for timingsafe_bcmp. Note that as far as I'm aware, it's
only available on OpenBSD.