Update from Gnulib

Update from Gnulib

This incorporates:
2020-05-03 attribute: new module
2020-04-13 explicit_bzero: improve code style
2020-04-13 explicit_bzero: On native Windows, use SecureZeroMemory
2020-04-13 explicit_bzero: use memset_s() when available
2020-04-04 maint: remove a stray inter-word space
* build-aux/config.guess, build-aux/config.sub:
* build-aux/gitlog-to-changelog, build-aux/update-copyright:
* doc/misc/texinfo.tex, lib/explicit_bzero.c, lib/ieee754.in.h:
* lib/nstrftime.c, m4/explicit_bzero.m4, m4/gnulib-common.m4:
Copy from Gnulib.
* lib/attribute.h: New file, copied from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.