build: switch to C++17

Change-Id: Ie68020a04b7e20b74778b6d0370544ded55c5e26
diff --git a/tests/tools/ndn-autoconfig/multicast-discovery.t.cpp b/tests/tools/ndn-autoconfig/multicast-discovery.t.cpp
index 9a0f5ac..b71cc84 100644
--- a/tests/tools/ndn-autoconfig/multicast-discovery.t.cpp
+++ b/tests/tools/ndn-autoconfig/multicast-discovery.t.cpp
@@ -62,7 +62,7 @@
       return;
     }
 
-    optional<ControlParameters> req = parseCommand(interest, "/localhost/nfd/rib/register");
+    auto req = parseCommand(interest, "/localhost/nfd/rib/register");
     if (req) {
       BOOST_REQUIRE(req->hasName());
       BOOST_CHECK_EQUAL(req->getName(), "/localhop/ndn-autoconf/hub");