build: switch to C++17

Change-Id: Ia147b22fbdee68d87f0289851683ffbbb4466caa
diff --git a/src/clients/response.hpp b/src/clients/response.hpp
index 4e8f335..5417c8c 100644
--- a/src/clients/response.hpp
+++ b/src/clients/response.hpp
@@ -33,8 +33,7 @@
 /**
  * @brief Default life time of resource record
  */
-const time::seconds DEFAULT_RR_FRESHNESS_PERIOD = 3600_s;
-
+inline constexpr time::seconds DEFAULT_RR_FRESHNESS_PERIOD = 3600_s;
 
 /**
  * @brief NDNS Response abstraction. Response is used on client side,