Avoid __GNUC__ warnings when defining DECLARE_DEPRECATED

We need to check that __GNUC__ is defined before trying to use it.
This demands a slightly different way to define DECLARE_DEPRECATED.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6688)