* src/bison.simple: Define type yystype instead of YYSTYPE, and

* src/bison.simple: Define type yystype instead of YYSTYPE, and
define CPP macro, which substitute YYSTYPE by yystype.
* src/reader.c (parse_union_decl): Output yystype/YYSTYPE as we do
with yyltype/YYLTYPE.  This allows inclusion of the generated
header within the parser if the compiler, such as GGC, accepts
multiple equivalent #defines.
From Akim.