stdckdint: use in parse-datetime

* lib/parse-datetime.y (apply_relative_time, zone, date)
(iso_8601_date, relunit, relunit_snumber, time_zone_hhmm)
(to_tm_year, yylex, parse_datetime_body):
Prefer ckd_add to INT_ADD_WRAPV etc., and include stdckdint.h.
* modules/parse-datetime (Depends-on): Add stdckdint.

modules/parse-datetime

@@ -22,6 +22,7 @@ inttypes
 mktime
 nstrftime
 setenv
+stdckdint
 unsetenv
 time-h
 time_r