AWS-LC is a BoringSSL-based libcrypto implementation. SHA_CTX is declared with
the hash data specified as an array rather than as a field in upstream OpenSSL.
Since AWS-LC builds against C99, we are unable to handle this with anonymous
unions like BoringSSL. The workaround I propose is to add these conditional
macros around the accessors within openssl_sha1_prf. After this change,
everything builds successfully with AWS-LC headers.
Closesstrongswan/strongswan#2103