Restore SSL_CTX_set_ecdh_auto compatibility hook.

This was inadvertently dropped in
59015c365b53a855513aaf5f9ff4597df9157ac0. 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>
diff --git a/PORTING.md b/PORTING.md
index 30d774c..b9d6752 100644
--- a/PORTING.md
+++ b/PORTING.md
@@ -227,6 +227,7 @@
 `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`