libstdc++: Fix localized %c formatting for non-UTC times [PR117214]

libstdc++: Fix localized %c formatting for non-UTC times [PR117214]

The previous commit fixed most cases of %c formatting, but it
incorrectly prints times using the system's local time zone. This only
matters if the locale's %c format includes %Z, but some do.

To print a correct value for %Z we can set tm.tm_zone to either "UTC" or
the abbreviation passed to the formatter in the local-time-format-t
structure.

For local times with no info and for systems that don't support tm_zone
(which is new in POSIX.1-2024) we just set tm_isdst = -1 so that no zone
name is printed.

In theory, a locale's %c format could use %z which should print a +hhmm
offset from UTC. I'm unsure how to control that though. The new
tm_gmtoff field in combination with tm_isdst != -1 seems like it should
work, but using that without also setting tm_zone causes the system zone
to be used for %Z again. That means local_time_format(lt, nullptr, &off)
might work for a locale that uses %z but prints the wrong thing for %Z.
This commit doesn't set tm_gmtoff even if _M_offset_sec is provided for
a local-time-format-t value.

libstdc++-v3/ChangeLog:

	PR libstdc++/117214
	* configure.ac: Use AC_STRUCT_TIMEZONE.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* include/bits/chrono_io.h (__formatter_chrono::_M_c): Set
	tm_isdst and tm_zone.
	* testsuite/std/time/format/pr117214.cc: Check %c formatting of
	zoned_time and local time.