Provide compat macros for SSL_CTX_set_ecdh_auto() and SSL_set_ecdh_auto()

These functions are no longer relevant to 1.1.0 (we always have auto ecdh
on) - but no reason to break old code that tries to call it. The macros will
only return a dummy "success" result if the app was trying to enable ecdh.
Disabling can't be done in quite this way any more.

Fixes Github Issue #1437

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>