This incorporates:
2019-07-06 thread, lock, cond, tls: Remove support for Pth threads
2019-07-02 verify: document ‘assume’ better
2019-06-30 Include <stdlib.h> when needed
2019-06-30 inet_ntop, inet_pton: Avoid conflict with native Windows
* build-aux/config.sub, lib/faccessat.c, lib/fcntl.c, lib/fstatat.c:
* lib/readlinkat.c, lib/verify.h, m4/pthread_sigmask.m4:
* m4/sys_socket_h.m4: Copy from Gnulib.
* lib/gnulib.mk.in: Regenerate.