Add commit 43aab01 of BenHanson/lexertl14 from github

YACReaderLibrary/lexertl/narrow.hpp

@@ -0,0 +1,25 @@
+// narrow.hpp
+// Copyright (c) 2015-2018 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_NARROW_HPP
+#define LEXERTL_NARROW_HPP
+
+#include <sstream>
+
+namespace lexertl
+{
+template<typename char_type>
+void narrow(const char_type *str_, std::ostringstream &ss_)
+{
+    while (*str_)
+    {
+        // Safe to simply cast to char.
+        // when string only contains ASCII.
+        ss_ << static_cast<char>(*str_++);
+    }
+}
+}
+
+#endif