Merge from Gnulib

Merge from Gnulib

This incorporates:
2017-10-29 timespec: prefer ‘assume’ to ‘assure’
2017-10-27 timespec.h: use "assure" to avoid a spurious warning
2017-10-09 getopt-posix: Fix build failure if ac_cv_header_getopt_h=no
* build-aux/config.guess, build-aux/config.sub:
* lib/timespec.h, lib/unistd.in.h:
Copy from Gnulib.