OPENSSL_API_COMPAT can be used to specify the OpenSSL API version in
use for the purpose of hiding deprecated interfaces and enabling
the appropriate deprecation notices.
This change is a NFC while we're still using OpenSSL 1.1.1 but will
avoid deprecation warnings upon the switch to OpenSSL 3.0.
A future change should migrate to use the OpenSSL 3.0 APIs, or just
remove libtelnet.
PR: 271615
Event: Kitchener-Waterloo Hackathon 202305
Sponsored by: The FreeBSD Foundation
ffbd1c8bc0