Previously, we were incorrectly assuming that the daylight global
variable indicated whether the current time zone is in DST. In reality,
the daylight variable only indicates whether a time zone *can* be in
DST.
Instead, the tm structure has a tm_isdst member that should be used for
this purpose. Ensure our LibC handles tm_isdst, and avoid errant usage
of the daylight variable in Core::DateTime.
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