This makes the NetBSD code thread-safe and presumably faster.
* lib/strftime.c (HAVE_NATIVE_TIME_Z): New macro.
(__strftime_internal): If HAVE_NATIVE_TIME_Z, use strftime_lz
or strftime_z instead of setting and reverting TZ.
* lib/time-internal.c: Remove, reverting recent change.
* lib/time_rz.c, modules/time_rz: Also revert recent changes,
since the relevant functions can now remain private to time_rz.c.
* m4/c-nstrftime.m4 (gl_C_GNU_STRFTIME): Check for strftime_lz.
* m4/nstrftime.m4 (gl_FUNC_GNU_STRFTIME): Check for strftime_z.