[PATCH 1/2] Merge mktime, timegm from upstream Gnulib

[Florian Weimer]

* Florian Weimer:

> * Paul Eggert:
>
>> [BZ #23603][BZ #16346]
>> This fixes some obscure problems with integer overflow.
>
> This patch breaks time/tst-mktime2 on armv7l-unknown-linux-gnueabihf,
> with GCC 7.3:
>
> tst-mktime2.c:70: numeric comparison failure
>    left: -1 (0xffffffff); from: mktime (lt)
>   right: 2147483647 (0x7fffffff); from: now
> tst-mktime2.c:70: numeric comparison failure
>    left: -1 (0xffffffff); from: mktime (lt)
>   right: -2147483648 (0x80000000); from: now
> tst-mktime2.c:70: numeric comparison failure
>    left: -1 (0xffffffff); from: mktime (lt)
>   right: 2147483646 (0x7ffffffe); from: now
> tst-mktime2.c:70: numeric comparison failure
>    left: -1 (0xffffffff); from: mktime (lt)
>   right: -2147483647 (0x80000001); from: now
> error: 4 test failures
>
> This is after patching the test like this:
>
>  65 static void
>  66 mktime_test1 (time_t now)
>  67 {
>  68   struct tm *lt = localtime (&now);
>  69   TEST_VERIFY_EXIT (lt != NULL);
>  70   TEST_COMPARE (mktime (lt), now);
>  71 }

It happens for the /usr/share/zoneinfo/America/Sao_Paulo file from
tzdata-2018e-1.fc27.  So the test case is not properly isolated, either.

Thanks,
Florian