I think I made a mistake during the port to OpenSSL 1.1.x. defined(OPENSSL_VERSION_1_1) is on the wrong ifndef block.

------------------------------------------------------------------
Old code

#ifndef OPENSSL_NO_ECDH
    /* Allow automatic ECDH curve selection (on OpenSSL 1.0.2+), or use
       prime256v1 by default.  This is Apache mod_ssl's initialization
       policy, so we should be safe. */
#if defined(SSL_CTX_set_ecdh_auto)
    SSL_CTX_set_ecdh_auto(self->ctx, 1);
#else
    {
        EC_KEY *key = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
        SSL_CTX_set_tmp_ecdh(self->ctx, key);
        EC_KEY_free(key);
    }
#endif
#endif

------------------------------------------------------------------
New code with OpenSSL 1.1.x compatibility

#ifndef OPENSSL_NO_ECDH
    /* Allow automatic ECDH curve selection (on OpenSSL 1.0.2+), or use
       prime256v1 by default.  This is Apache mod_ssl's initialization
       policy, so we should be safe. OpenSSL 1.1 has it enabled by default.
     */
#if defined(SSL_CTX_set_ecdh_auto) && !defined(OPENSSL_VERSION_1_1)
    SSL_CTX_set_ecdh_auto(self->ctx, 1);
#else
    {
        EC_KEY *key = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
        SSL_CTX_set_tmp_ecdh(self->ctx, key);
        EC_KEY_free(key);
    }
#endif
#endif