Problems reported by Tom G. Christensen in:
https://lists.gnu.org/r/bug-gnulib/2017-11/msg00053.html
* doc/glibc-functions/strverscmp.texi (strverscmp):
Document strverscmp bug with glibc 2.9 and earlier.
* doc/posix-functions/tzset.texi (tzset):
Document that TZ with angle brackets is POSIX-2001 and later.
* tests/test-nstrftime.c: Include unistd.h.
(TZ_ANGLE_BRACKETS_SHOULD_WORK): New macro.
(TZ): Use it to skip tests with angle brackets in TZ,
for older systems.