Add commit 43aab01 of BenHanson/lexertl14 from github

YACReaderLibrary/lexertl/observer_ptr.hpp

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+// observer_ptr.hpp
+// Copyright (c) 2017-2018 Ben Hanson (http://www.benhanson.net/)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file licence_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef LEXERTL_OBSERVER_PTR_HPP
+#define LEXERTL_OBSERVER_PTR_HPP
+
+namespace lexertl
+{
+    template<typename T>
+    using observer_ptr = T *;
+}
+
+#endif