Bison 3.1 api.value.type {semantic_type} error in gcc8

[Hans Åberg]

In Bison 3.1,
  %define api.value.type {semantic_type}
produces an error in gcc8, though accepted in clang6, by the parser header 
typedef
  #ifndef YYSTYPE
    /// Symbol semantic values.
    typedef semantic_type semantic_type;
  #else
    typedef YYSTYPE semantic_type;
  #endif