build: switch to C++17

Change-Id: Ie68020a04b7e20b74778b6d0370544ded55c5e26
diff --git a/tests/daemon/mgmt/rib-manager.t.cpp b/tests/daemon/mgmt/rib-manager.t.cpp
index d23eb70..682eb30 100644
--- a/tests/daemon/mgmt/rib-manager.t.cpp
+++ b/tests/daemon/mgmt/rib-manager.t.cpp
@@ -448,7 +448,7 @@
     rib::Route route;
     route.faceId = ++faceId;
     route.cost = route.faceId * 10;
-    route.expires = nullopt;
+    route.expires = std::nullopt;
     return route;
   };