Update from Gnulib

Update from Gnulib

This incorporates:
2020-05-09 stdio: don't redefine _GL_ATTRIBUTE_FORMAT
2020-05-09 dirent, stdlib, string: don't redefine _GL_ATTRIBUTE_PURE
2020-05-08 limits-h: define LONG_BIT correctly on Haiku/x86_64
2020-05-08 ignore-value tests: use module 'attribute'
2020-05-06 attribute: minor style fixes
* build-aux/config.sub, doc/misc/texinfo.tex, lib/attribute.h:
* lib/dirent.in.h, lib/limits.in.h, lib/stdio.in.h, lib/stdlib.in.h:
* lib/string.in.h, m4/gnulib-common.m4:
Copy from Gnulib.