BUILD: makefile: silence unbearable OpenSSL deprecation warnings

BUILD: makefile: silence unbearable OpenSSL deprecation warnings

OpenSSL 3.0 emits tons of deprecation warnings for the engine API, and
it becomes a real problem because these hide other real warnings and
will prevent distros from building with -Werror. Fortunately there's a
macro to shut this one, OPENSSL_SUPPRESS_DEPRECATED, that is sufficient
to get things back to normal, so let's define it when USE_ENGINE is set.
This way we still get a chance to see other deprecation warnings when
engines are not used.