When parse-datetime.y’s overflow code was written, INT_ADD_WRAPV
did not work for unsigned destinations, and since time_t might
be unsigned that meant it did not work for time_t destinations.
This limitation of INT_ADD_WRAPV has been fixed, so we can
now streamline parse-datetime.y a bit.
* lib/parse-datetime.y: Do not include limits.h, as LONG_MAX
has not been used for a while.
(yylex, parse_datetime2): Assume C99 declarations after statements.
(yyles): Use INT_SUBTRACT_WRAPV instead of an explicit comparison
to TYPE_MINIMUM.
(parse_datetime2): No need for time_overflow now that
INT_ADD_WRAPV works for unsigned results.