Move code around to make a merge to glibc easier to audit.
This should not change behavior.
* lib/mktime.c: Include more standard files unconditionally.
(NEED_MKTIME_INTERNAL, NEED_MKTIME_WINDOWS)
(NEED_MKTIME_WORKING): Give default values to pacify -Wundef,
which glibc uses. Default NEED_MKTIME_WORKING to DEBUG_MKTIME, to
simplify later conditionals; default the others to zero. In uses
of these conditionals, explicitly spell out how _LIBC affects
things, so it’s easier to review from a glibc viewpoint.
(my_tzset, __tzset) [!_LIBC]: New function and macro, to better
compartmentalize tzset issues. Move system-dependent tzsettish
code here from mktime.
(mktime): Move tzsettish code to my_tzset, and move
localtime_offset to within mktime so that it doesn’t
need a separate ifdef.
* lib/mktime-internal.h (__gmtime_r, __localtime_r, __mktime_internal):
Move these macros here from lib/mktime.c and lib/gmtime.c.