Use autoconf check for BN_GENCB_new()

Use autoconf check for BN_GENCB_new()

BIND unconditionally uses shims for BN_GENCB_new(), BN_GENCB_free(),
and BN_GENCB_get_arg() for all LibreSSL versions and, correctly, for
OpenSSL <1.1.0 versions.

This breaks LibreSSL compilation starting with LibreSSL 3.5.0.

Use autoconf check instead to check whether the family of the functions
are available.

(cherry picked from commit 749973f3259b7638a6af02b7da2f40ae28bdd402)