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This incorporates:
2020-02-23 use 'restrict'
* lib/careadlinkat.h, lib/md5.h, lib/sha1.h, lib/sha256.h:
* lib/sha512.h, lib/strftime.h, lib/string.in.h, m4/nstrftime.m4:
Copy from Gnulib.
* m4/gnulib-comp.m4: Regenerate.