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This incorporates:
2018-11-15 mktime: DEBUG_MKTIME cleanup
2018-11-15 mktime: fix non-EOVERFLOW errno handling
2018-11-15 mktime: fix bug with Y2038 DST transition
2018-11-15 mktime: make more room for overflow
2018-11-15 mktime: simplify offset guess
2018-11-15 mktime: new test for mktime failure
2018-11-15 mktime: fix EOVERFLOW bug
2018-11-13 longlong: fix comment typo
* lib/gnulib.mk.in: Regenerate.
* lib/mktime.c, m4/longlong.m4: Copy from Gnulib.