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.
BIP324 mentions K1 is used for the associated data and K2 is used for
the payload. The code does the opposite. This is not a security problem
but will be a problem across implementations based on the HKDF key
derivations.
