api.prefix: do not use #define to handle YYSTYPE_IS_TRIVIAL etc.

api.prefix: do not use #define to handle YYSTYPE_IS_TRIVIAL etc.

The following mixture is insane:

  #define YYSTYPE_IS_TRIVIAL PREFIX_STYPE_IS_TRIVIAL
  #if (defined YYSTYPE_IS_TRIVIAL && YYSTYPE_IS_TRIVIAL)

since, of course YYSTYPE_IS_TRIVIAL is defined.  Instead we could
define YYSTYPE_IS_TRIVIAL as PREFIX_STYPE_IS_TRIVIAL only when the
later is defined, but let's avoid stacking CPP on top of M4: rather, use

  #if (defined PREFIX_STYPE_IS_TRIVIAL && PREFIX_STYPE_IS_TRIVIAL)

* data/glr.c, data/yacc.c: Use YYSTYPE_IS_TRIVIAL, YYSTYPE_IS_DECLARED,
YYLTYPE_IS_TRIVIAL and YYLTYPE_IS_DECLARED under their api.prefix-renamed
name.