(union yyalloc): Do not evaluate

YYSTYPE_IS_TRIVIAL or YYSTYPE_IS_TRIVIAL unless they are defined.
This fixes a problem reported by John Bowman when the Compaq/HP
Alpha cxx compiler happy (e.g. using cxx -D__USE_STD_IOSTREAM
-ansi -Wall -gall).
(YYCOPY): Do not evaluate __GNUC__ unless it is defined.