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This incorporates:
2018-12-16 regex: propagate fix for glibc bug 18040
2018-12-16 obstack, libc-config: Support HP-UX cc in C99 mode
2018-12-15 regex: work around a bug in glibc-2.27 and prior
2018-12-13 localtime-buffer: Avoid endless recursion
* build-aux/config.guess, build-aux/config.sub, lib/cdefs.h:
* lib/gettext.h, lib/localtime-buffer.c, lib/regexec.c, m4/regex.m4:
Copy from Gnulib.