build: switch to C++17

Change-Id: Ie68020a04b7e20b74778b6d0370544ded55c5e26
diff --git a/daemon/rib/readvertise/readvertise.cpp b/daemon/rib/readvertise/readvertise.cpp
index 2f16795..addad72 100644
--- a/daemon/rib/readvertise/readvertise.cpp
+++ b/daemon/rib/readvertise/readvertise.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2019,  Regents of the University of California,
+ * Copyright (c) 2014-2022,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -67,7 +67,7 @@
 void
 Readvertise::afterAddRoute(const RibRouteRef& ribRoute)
 {
-  optional<ReadvertiseAction> action = m_policy->handleNewRoute(ribRoute);
+  std::optional<ReadvertiseAction> action = m_policy->handleNewRoute(ribRoute);
   if (!action) {
     NFD_LOG_DEBUG("add-route " << ribRoute.entry->getName() << '(' << ribRoute.route->faceId <<
                   ',' << ribRoute.route->origin << ") not-readvertising");