add libcurl min version

Author: scotthart

google/cloud/credentials.h

@@ -39,6 +39,8 @@ namespace experimental {
  *
  * @note This option is currently experimental and only works with services
  *     using JSON/HTTP transport.
+ *
+ * @note Requires libcurl v7.71.0 or later.
  */
 struct ClientSslCertificateOption {
   using Type = SslCertificate;

google/cloud/internal/curl_impl.cc

@@ -333,6 +333,7 @@ Status CurlImpl::MakeRequest(HttpMethod method, RestContext& context,
     if (!status.ok()) return OnTransferError(context, std::move(status));
   }
 
+#if CURL_AT_LEAST_VERSION(7, 71, 0)
   if (client_ssl_cert_.has_value()) {
     status = handle_.SetOption(CURLOPT_SSL_VERIFYPEER, 1L);
     if (!status.ok()) return OnTransferError(context, std::move(status));
@@ -361,6 +362,7 @@ Status CurlImpl::MakeRequest(HttpMethod method, RestContext& context,
     status = handle_.SetOption(CURLOPT_SSLKEY_BLOB, &ssl_key_blob);
     if (!status.ok()) return OnTransferError(context, std::move(status));
   }
+#endif
 
   if (method == HttpMethod::kGet) {
     status = handle_.SetOption(CURLOPT_NOPROGRESS, 1L);