Merge from gnulib

Merge from gnulib

This incorporates:
2017-05-13 largefile: Simplify
2017-05-13 largefile: Improve and document
2017-05-13 truncate: New module
2017-05-13 windows-stat-timespec: New module
2017-05-13 windows-stat-override: New module
2017-05-11 getopt-posix: port to mingw
2017-05-11 gettimeofday: Increase precision on mingw
2017-05-10 time: Fix missing initialization of HAVE_TIMEZONE_T
2017-05-10 Implement a way to opt out from MSVC support
2017-05-09 tzset: Expand comment about TZ problem on native Windows
* build-aux/config.guess, lib/dup2.c, lib/fcntl.c, lib/fsync.c:
* lib/getdtablesize.c, lib/getopt.c, lib/gettimeofday.c:
* lib/mktime.c, lib/stat-time.h, lib/sys_stat.in.h, lib/unistd.in.h:
* lib/utimens.c, m4/gettimeofday.m4, m4/largefile.m4:
* m4/sys_stat_h.m4, m4/sys_time_h.m4, m4/time_h.m4, m4/time_rz.m4:
* m4/unistd_h.m4: Copy from gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.