Add commit 43aab01 of BenHanson/lexertl14 from github

YACReaderLibrary/lexertl/runtime_error.hpp

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+// runtime_error.hpp
+// Copyright (c) 2005-2018 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_RUNTIME_ERROR_HPP
+#define LEXERTL_RUNTIME_ERROR_HPP
+
+#include <stdexcept>
+
+namespace lexertl
+{
+class runtime_error : public std::runtime_error
+{
+public:
+    runtime_error(const std::string &what_arg_) :
+        std::runtime_error(what_arg_)
+    {
+    }
+};
+}
+
+#endif