Add OPENSSL_NO_ECDH guards

Reviewed-by: Emilia Käsper <emilia@openssl.org>
(cherry picked from commit af6e2d51bfeabbae827030d4c9d58a8f7477c4a0)
Add more meaningful OPENSSL_NO_ECDH error message for suite b mode

Reviewed-by: Emilia Käsper <emilia@openssl.org>
(cherry picked from commit db812f2d70f0695fd53b386fe5e870bef8ca3c22)