ssl: call SSL_CTX_set_ecdh_auto() on OpenSSL 1.0.2 only

SSL_CTX_set_ecdh_auto() exists in OpenSSL 1.1.0 and LibreSSL 2.6.1, but
it is made no-op and the automatic curve selection cannot be disabled.
Wrap it with ifdef to make it clear that it is safe to remove it
completely when we drop support for OpenSSL 1.0.2.

ext/openssl/ossl_ssl.c

@@ -93,14 +93,15 @@ ossl_sslctx_s_alloc(VALUE klass)
     RTYPEDDATA_DATA(obj) = ctx;
     SSL_CTX_set_ex_data(ctx, ossl_sslctx_ex_ptr_idx, (void *)obj);
 
-#if !defined(OPENSSL_NO_EC)
+#if !defined(OPENSSL_NO_EC) && OPENSSL_VERSION_NUMBER < 0x10100000 && \
+    !defined(LIBRESSL_VERSION_NUMBER)
     /* We use SSL_CTX_set1_curves_list() to specify the curve used in ECDH. It
      * allows to specify multiple curve names and OpenSSL will select
      * automatically from them. In OpenSSL 1.0.2, the automatic selection has to
-     * be enabled explicitly. But OpenSSL 1.1.0 removed the knob and it is
-     * always enabled. To uniform the behavior, we enable the automatic
-     * selection also in 1.0.2. Users can still disable ECDH by removing ECDH
-     * cipher suites by SSLContext#ciphers=. */
+     * be enabled explicitly. OpenSSL 1.1.0 and LibreSSL 2.6.1 removed the knob
+     * and it is always enabled. To uniform the behavior, we enable the
+     * automatic selection also in 1.0.2. Users can still disable ECDH by
+     * removing ECDH cipher suites by SSLContext#ciphers=. */
     if (!SSL_CTX_set_ecdh_auto(ctx, 1))
 	ossl_raise(eSSLError, "SSL_CTX_set_ecdh_auto");
 #endif