Update from Gnulib.

Update from Gnulib.

This incorporates:
2019-03-23 Support cross-compilation to musl libc
2019-03-23 noreturn: In C++ mode with clang, use _Noreturn as fallback
2019-03-22 _Noreturn: beware of C's _Noreturn in C++ pre C++11
2019-03-19 Help making signal handlers more reliable
2019-03-18 _Noreturn: clang and MSVC do support [[noreturn]] in C++11
2019-03-17 _Noreturn: GCC 4.7 does not support [[noreturn]] in C++11
2019-03-14 all: Update URLs to msdn.microsoft.com
* doc/misc/texinfo.tex, lib/_Noreturn.h, lib/gettimeofday.c:
* lib/mktime.c, lib/regcomp.c, lib/regexec.c, lib/stat-time.h:
* lib/utimens.c, m4/fdopendir.m4, m4/getgroups.m4:
* m4/gettimeofday.m4, m4/gnulib-common.m4, m4/putenv.m4, m4/utimes.m4:
Update from gnulib.