Disable parse_config_file specialization for wchar_t if we don't have wstring.

[SVN r22925]

src/parsers.cpp

@@ -267,10 +267,12 @@ namespace boost { namespace program_options {
     parse_config_file(std::basic_istream<char>& is, 
                       const options_description& desc);
 
+#ifndef BOOST_NO_STD_WSTRING
     template
     basic_parsed_options<wchar_t>
     parse_config_file(std::basic_istream<wchar_t>& is, 
                       const options_description& desc);
+#endif
     
 // This versio, which accepts any options without validation, is disabled,
 // in the hope that nobody will need it and we cant drop it altogether.