parse-datetime: don’t depend on tzname

* lib/parse-datetime.y (TIME_ZONE_BUFSIZE): Move earlier.
(parser_control) [!HAVE_STRUCT_TM_TM_ZONE]:
New member tz_abbr, to save abbrs calculated by strftime %Z.
(populate_local_time_zone_table): New function, which
optimizes the HAVE_STRUCT_TM_TM_ZONE case as before,
and falls back on strftime with %Z otherwise.
Although strftime %Z can be more accurate than the old tzname
based method, the new heuristic is still wrong so often that it
probably doesn’t help all that much.
(parse_datetime_body): Use it.
* modules/parse-datetime (Depends-on): Remove tzname.

modules/parse-datetime

@@ -5,8 +5,8 @@ Files:
 doc/parse-datetime.texi
 lib/parse-datetime.h
 lib/parse-datetime.y
-m4/tm_gmtoff.m4
 m4/parse-datetime.m4
+m4/tm_gmtoff.m4
 
 Depends-on:
 assert-h
@@ -28,7 +28,6 @@ time-h
 time_r
 time_rz
 timegm
-tzname
 
 configure.ac:
 gl_PARSE_DATETIME