tests: address g++-4.8 warnings.

list.yy: In function 'yy::parser::symbol_type yylex()':
list.yy:107:29: error: typedef 'token' locally defined but not used
  [-Werror=unused-local-typedefs]
   typedef yy::parser::token token;
                             ^

* tests/c++.at (AT_CHECK_VARIANTS): here.

tests/c++.at

@@ -127,7 +127,9 @@ yy::parser::token_type yylex(yy::parser::semantic_type* yylval,
                              yy::parser::location_type* yylloc)
 #endif
 {
+#ifndef USE_LEX_SYMBOL
   typedef yy::parser::token token;
+#endif
   typedef yy::parser::location_type location_type;
   static int stage = -1;
   ++stage;