build: raise minimum build requirements

 * Make gcc 7.4 and boost 1.65.1 hard dependencies, both are
   available out-of-the-box on Ubuntu 18.04
 * Require Xcode 10 on macOS but recommend 11.3 or later
 * If using clang on non-Apple platforms, the minimum supported
   version is now 6.0

Change-Id: I79c84a266741c9a66655da4fd467fe0d8218e8f0
diff --git a/tools/ndnsec/accumulator.hpp b/tools/ndnsec/accumulator.hpp
index fb7f357..a696000 100644
--- a/tools/ndnsec/accumulator.hpp
+++ b/tools/ndnsec/accumulator.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2021 Regents of the University of California.
+ * Copyright (c) 2013-2022 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -69,7 +69,7 @@
   std::string
   name() const final
   {
-    return std::string();
+    return {};
   }
 
   // There are no tokens for an AccumulatorType
@@ -134,14 +134,6 @@
       *m_store = *val;
   }
 
-#if (BOOST_VERSION >= 105900) && (BOOST_VERSION < 106500)
-  bool
-  adjacent_tokens_only() const final
-  {
-    return false;
-  }
-#endif // (BOOST_VERSION >= 105900) && (BOOST_VERSION < 106500)
-
 private:
   T* m_store;
   T m_interval;