build: switch to C++17

Change-Id: Ie68020a04b7e20b74778b6d0370544ded55c5e26
diff --git a/daemon/rib/readvertise/host-to-gateway-readvertise-policy.hpp b/daemon/rib/readvertise/host-to-gateway-readvertise-policy.hpp
index 325fc80..ac2491d 100644
--- a/daemon/rib/readvertise/host-to-gateway-readvertise-policy.hpp
+++ b/daemon/rib/readvertise/host-to-gateway-readvertise-policy.hpp
@@ -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,
@@ -43,7 +43,7 @@
                                  const ConfigSection& section);
 
 public:
-  optional<ReadvertiseAction>
+  std::optional<ReadvertiseAction>
   handleNewRoute(const RibRouteRef& ribRoute) const override;
 
   time::milliseconds