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This incorporates:
2016-05-01 mktime: port to stricter signed overflow checking
2016-05-01 mktime: speed up DEBUG_MKTIME benchmarks
2016-05-01 mktime: resurrect DEBUG_MKTIME testing
2016-05-01 mktime: simplify DEBUG_MKTIME
2016-05-01 Port mktime_internal offset to unsigned time_t
2016-04-27 xstrtol: prohibit monstrosities like "1bB"
2016-04-13 mktime: improve integer overflow checking
2016-04-13 intprops: check two's complement assumption
2016-04-13 intprops, mktime, strtol: assume two's complement
* lib/intprops.h, lib/mktime-internal.h, lib/mktime.c:
* lib/strtol.c, lib/timegm.c, m4/mktime.m4, m4/std-gnu11.m4:
Copy from gnulib.