LibreSSL v2.7 claims an OPENSSL_VERSION_NUMBER value that would indicate
that SSL_OP_NO_TLSv1_3 is available, but that does not seem to be the
case with LibreSSL. As such, skip this step based on whether
SSL_OP_NO_TLSv1_3 is defined to avoid build issues.
Signed-off-by: Jouni Malinen <j@w1.fi>