Update from Gnulib

Update from Gnulib

This incorporates:
2019-05-24 flexmember: update comments
2019-05-18 pthread_sigmask: fix --enable-threads=windows compilation
2019-05-14 close-stream, closein, closeout: simplify
2019-05-09 verify: remove verify_true
2019-05-09 verify: support C2X and C++17 static_assert
* build-aux/config.guess, build-aux/config.sub:
* doc/misc/texinfo.tex, lib/flexmember.h, lib/verify.h:
* m4/flexmember.m4, m4/pthread_sigmask.m4:
Copy from Gnulib
* m4/gnulib-comp.m4: Regenerate.

2019-05-26  Paul Eggert  <eggert@cs.ucla.edu>

Update author/maintainer info
This mostly updates email addresses and fixes spellings of
author and maintainer names.