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This incorporates:
2020-02-08 lchmod: ensure declaration on HP-UX
2020-02-08 fchmodat: fix endless recursion on Cygwin
2020-02-08 Fix compilation errors in a testdir
2020-02-07 fchmodat: AT_SYMLINK_NOFOLLOW fix for non-symlinks
2020-02-04 Port _Noreturn to older Clang
2020-02-03 libc-config: port to Apple’s Clang variant
* lib/_Noreturn.h, lib/c++defs.h, lib/libc-config.h, lib/sys_stat.in.h:
* m4/gnulib-common.m4, m4/sys_stat_h.m4: Copy from Gnulib.
* lib/gnulib.mk.in: Regenerate.