This incorporates:
2015-07-27 time_rz: port better to MinGW
2015-07-27 time: port __need_time_t to MinGW
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/strftime.c, lib/time.in.h, lib/time_rz.c: Copy from gnulib.
* lib/time-internal.h: New file, from gnulib.