Update default compilation flags

Synced from ndn-tools

Change-Id: I95e451f6978f473ad6f616277803464effffd3a7
diff --git a/src/challenge-module.hpp b/src/challenge-module.hpp
index af0d40b..b2d39ad 100644
--- a/src/challenge-module.hpp
+++ b/src/challenge-module.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2017, Regents of the University of California.
+ * Copyright (c) 2017-2018, Regents of the University of California.
  *
  * This file is part of ndncert, a certificate management system based on NDN.
  *
@@ -44,8 +44,12 @@
   };
 
 public:
+  explicit
   ChallengeModule(const std::string& uniqueType);
 
+  virtual
+  ~ChallengeModule();
+
   template<class ChallengeType>
   static void
   registerChallengeModule(const std::string& typeName)