Avoid __GNUC__ warnings when defining DECLARE_DEPRECATED
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: Rich Salz <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6680)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl