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Following an earlier fix, the logic of mktime() has turned out to need reviewing too.
To summarise
  time() -> a UTC time in seconds
  gmtime() -> breaks down a UTC time into components
  localtime() -> breaks down a UTC time into local components (tz + DST)
  mktime() -> converts local components back into UTC time, normalising
In mktime(), the current locale (via setlocale()) is considered and used to choose the timezone, assuming an appropriate territory module is loaded.
The value of the tm_isdst field is factored into the offset calculation.
However, mktime() is also defined as normalising the components of time and updating the caller's block with normalised values, in local time, so the local offset is reapplied at the end.

Moved time_to_tm up in the function so mktime() can use it.
Corrected/lined up some comments.
Also, mktime() no longer forces tm_isdst to -1, it preserves the user's value, so consecutive mktime()s are consistent.
Added test to "test/tzones...