LibreSSL's fork does not have this new function; as well as adding a
`LIBRESSL_VERSION_NUMBER` value, that project bumped the OpenSSL version
number in such a way as to conflict with our existing version checks.
* Add a guard.
* Add commentary, suggesting how to avoid getting into twistier knots
with API divergence.
Reported by Jasper Wallace, who provided a slightly different patch.
Fixes bug 1635