Restore SSL_CTX_set_ecdh_auto compatibility hook.

This was inadvertently dropped in
59015c365b. Python otherwise configures
P-256 if it assumes our OpenSSL predate's 1.0.2's multi-curve support.
This disables X25519, our preferred curve.

Change-Id: Ibf758583ea53e68c56667f16ee7096656bac719b
Reviewed-on: https://boringssl-review.googlesource.com/14208
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

PORTING.md

@@ -227,6 +227,7 @@ parameter.
 `SSL_CTRL_OPTIONS` | `SSL_CTX_get_options` or `SSL_CTX_set_options`
 `SSL_CTRL_SESS_NUMBER` | `SSL_CTX_sess_number`
 `SSL_CTRL_SET_CURVES` | `SSL_CTX_set1_curves`
+`SSL_CTRL_SET_ECDH_AUTO` | `SSL_CTX_set_ecdh_auto`
 `SSL_CTRL_SET_MAX_CERT_LIST` | `SSL_CTX_set_max_cert_list`
 `SSL_CTRL_SET_MAX_SEND_FRAGMENT` | `SSL_CTX_set_max_send_fragment`
 `SSL_CTRL_SET_MSG_CALLBACK` | `SSL_set_msg_callback`

include/openssl/ssl.h

@@ -3800,6 +3800,12 @@ OPENSSL_EXPORT const BIO_METHOD *BIO_f_ssl(void);
  * other than one on error. */
 OPENSSL_EXPORT long BIO_set_ssl(BIO *bio, SSL *ssl, int take_owership);
 
+/* SSL_CTX_set_ecdh_auto returns one. */
+#define SSL_CTX_set_ecdh_auto(ctx, onoff) 1
+
+/* SSL_set_ecdh_auto returns one. */
+#define SSL_set_ecdh_auto(ssl, onoff) 1
+
 
 /* Private structures.
  *
@@ -4294,6 +4300,7 @@ struct ssl_ctx_st {
 #define SSL_CTRL_SESS_NUMBER doesnt_exist
 #define SSL_CTRL_SET_CURVES doesnt_exist
 #define SSL_CTRL_SET_CURVES_LIST doesnt_exist
+#define SSL_CTRL_SET_ECDH_AUTO doesnt_exist
 #define SSL_CTRL_SET_MAX_CERT_LIST doesnt_exist
 #define SSL_CTRL_SET_MAX_SEND_FRAGMENT doesnt_exist
 #define SSL_CTRL_SET_MSG_CALLBACK doesnt_exist