By now, we have several locations where we are checking the version of
OpenSSL to determine whether we can use the new "modern" API or need to
use the pre-1.1 legacy API. As we have multiple implementations of
OpenSSL with the rather recent libressl implementation, these checks
need to honor versions of both implementations, which is rather tedious.
Instead, we can just check once for the correct versions and define
`OPENSSL_LEGACY_API` in case we cannot use the modern API.
