build: add OPENSSL_API_COMPAT macro

This commit adds the OPENSSL_API_COMPAT macro and sets it to version
1.1.1 of OpenSSL when linking with a shared OpenSSL library.

The motivation for this is that when linking against OpenSSL 3.x there
are a lot of deprecation warnings and this allows them to be avoided.
When we later upgrade the code base to 3.x this value can then be
updated.