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.

lib/mktime.c

@@ -491,9 +491,28 @@ time_t
 mktime (struct tm *tp)
 {
 # if NEED_MKTIME_WINDOWS
-  /* If the environment variable TZ has been set by Cygwin, neutralize it.
-     The Microsoft CRT interprets TZ differently than Cygwin and produces
-     incorrect results if TZ has the syntax used by Cygwin.  */
+  /* Rectify the value of the environment variable TZ.
+     There are four possible kinds of such values:
+       - Traditional US time zone names, e.g. "PST8PDT".  Syntax: see
+         <https://msdn.microsoft.com/en-us/library/90s5c885.aspx>
+       - Time zone names based on geography, that contain one or more
+         slashes, e.g. "Europe/Moscow".
+       - Time zone names based on geography, without slashes, e.g.
+         "Singapore".
+       - Time zone names that contain explicit DST rules.  Syntax: see
+         <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03>
+     The Microsoft CRT understands only the first kind.  It produces incorrect
+     results if the value of TZ is of the other kinds.
+     But in a Cygwin environment, /etc/profile.d/tzset.sh sets TZ to a value
+     of the second kind for most geographies, or of the first kind in a few
+     other geographies.  If it is of the second kind, neutralize it.  For the
+     Microsoft CRT, an absent or empty TZ means the time zone that the user
+     has set in the Windows Control Panel.
+     If the value of TZ is of the third or fourth kind -- Cygwin programs
+     understand these syntaxes as well --, it does not matter whether we
+     neutralize it or not, since these values occur only when a Cygwin user
+     has set TZ explicitly; this case is 1. rare and 2. under the user's
+     responsibility.  */
   const char *tz = getenv ("TZ");
   if (tz != NULL && strchr (tz, '/') != NULL)
     _putenv ("TZ=");