ldns: specify OpenSSL 1.1 APIs

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 ldns update should migrate to use the OpenSSL 3.0 APIs.

This is a recommit of c6750ddec917 after the ldns update in
5afab0e5e56f.

PR: 271615
Event: Kitchener-Waterloo Hackathon 202305
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40082