Retain the possibility to use OpenSSL engines with OpenSSL 3.x
See #2996 for context.
It would be useful to retain the possibility to use OpenSSL engines with
OpenSSL 3.x; it is possible to build against the latter in API
compatibility mode (-DOPENSSL_API_COMPAT=10100
), so we should take
advantage of that before a "native" OpenSSL 3.x provider gets
implemented.