Update from Gnulib

Update from Gnulib

This incorporates:
2020-08-20 sigabbrev_np: New module
2020-08-20 stdalign: Fix 32-bit test failures clang versions < 8
2020-08-17 careadlinkat: speedup for GCC 10 with GCC_LINT
2020-08-17 Assume autoconf >= 2.64
* build-aux/config.guess, build-aux/config.sub, lib/careadlinkat.c:
* lib/stdalign.in.h, lib/string.in.h, m4/std-gnu11.m4, m4/string_h.m4:
Copy from Gnulib.
* lib/gnulib.mk.in: Regenerate.