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: NViktor Dukhovni <viktor@openssl.org>

include/openssl/ssl.h

@@ -1279,6 +1279,8 @@ DECLARE_PEM_rw(SSL_SESSION, SSL_SESSION)
 # define SSL_CTX_set_tmp_rsa(ctx,rsa)             1
 # define SSL_need_tmp_RSA(ssl)                    0
 # define SSL_set_tmp_rsa(ssl,rsa)                 1
+# define SSL_CTX_set_ecdh_auto(dummy, onoff)      ((onoff) != 0)
+# define SSL_set_ecdh_auto(dummy, onoff)          ((onoff) != 0)
 /*
  * We "pretend" to call the callback to avoid warnings about unused static
  * functions.