sys_time: Override 'struct timeval' on some native Windows platforms.

sys_time: Override 'struct timeval' on some native Windows platforms.

* m4/sys_time_h.m4 (gl_HEADER_SYS_TIME_H_BODY): Test whether tv_sec
has the right type. Set REPLACE_STRUCT_TIMEVAL if not.
(gl_HEADER_SYS_TIME_H_DEFAULTS): Initialize REPLACE_STRUCT_TIMEVAL.
* lib/sys_time.in.h: Include <winsock2.h> also when 'struct timeval'
needs to be overridden.
(timeval): Override if REPLACE_STRUCT_TIMEVAL is set.
* modules/sys_time (Makefile.am): Substitute REPLACE_STRUCT_TIMEVAL.
* tests/test-sys_select.c: Check that the tv_sec member has the same
size as a 'time_t'.
* tests/test-sys_time.c: Likewise.
* m4/gettimeofday.m4 (gl_FUNC_GETTIMEOFDAY): If REPLACE_STRUCT_TIMEVAL
is set, set also REPLACE_GETTIMEOFDAY.
* lib/gettimeofday.c (gettimeofday): If 'struct timeval' is overridden,
convert the resulting 'struct timeval' before returning.
* lib/select.c: Include <sys/time.h>.
(select, timeval): Undefine at the right place.
* modules/select (Depends-on): Add sys_time.
* doc/posix-headers/sys_time.texi: Mention the problem with tv_sec on
some Windows platforms.
Reported by Marc-André Lureau <marcandre.lureau@redhat.com>.