fix issue of fold expressions test(#320,#321)

source/gui/msgbox.cpp

@@ -1,7 +1,7 @@
 /*
  *	A Message Box Class
  *	Nana C++ Library(http://www.nanapro.org)
- *	Copyright(C) 2003-2017 Jinhao(cnjinhao@hotmail.com)
+ *	Copyright(C) 2003-2018 Jinhao(cnjinhao@hotmail.com)
  *
  *	Distributed under the Boost Software License, Version 1.0.
  *	(See accompanying file LICENSE_1_0.txt or copy at
@@ -1294,7 +1294,7 @@ namespace nana
         min_width_entry_field_pixels_ = pixels;
 	}
 
-#ifndef _nana_cxx_folding_expression
+#ifndef __cpp_fold_expressions
 	void inputbox::_m_fetch_args(std::vector<abstract_content*>&)
 	{}
 #endif