MS-Windows follow-up for recent TZ-related changes

MS-Windows follow-up for recent TZ-related changes

* nt/mingw-cfg.site (ac_cv_header_pthread_h)
(gl_cv_sys_struct_timespec_in_pthread_h): Force to "no", to avoid
picking up 'struct timespec' from pthread.h, if it is installed on
the user's system.  We want either the definitions from MinGW
system headers, if available, or the Gnulib replacements if not.

* nt/inc/ms-w32.h <struct timespec>: Don't define, as we now use
lib/time.h.

* lib/time.in.h: Don't let __need_* symbols affect what happens on
MinGW.  These symbols are defined by MinGW system headers, but we
don't want that to affect whether Gnulib portions of the header
are or aren't used.