build: recommend boost >= 1.65.1

With this commit, Ubuntu 16.04 is no longer officially supported
and may require manual installation of a newer version of Boost.

Refs: #5087
Change-Id: I17197f72ee8f7cd4d0954658fe57c324076f1311
diff --git a/tools/ndnsec/export.cpp b/tools/ndnsec/export.cpp
index a11459d..6d3c260 100644
--- a/tools/ndnsec/export.cpp
+++ b/tools/ndnsec/export.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2019 Regents of the University of California.
+ * Copyright (c) 2013-2020 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -38,6 +38,8 @@
   std::string output;
   std::string password;
 
+  // ASan reports a stack-use-after-scope on armhf when
+  // BOOST_SCOPE_EXIT_ALL is used in place of BOOST_SCOPE_EXIT
   BOOST_SCOPE_EXIT(&password) {
     OPENSSL_cleanse(&password.front(), password.size());
   } BOOST_SCOPE_EXIT_END