gettimeofday: port recent C++ fix to Emacs

gettimeofday: port recent C++ fix to Emacs

Without this further patch, Emacs won't build due to
the portcheck failing.  Also, this simplifies the patch a bit.
* lib/time.in.h (localtime, gmtime): Don't replace unless
GNULIB_GETTIMEOFDAY.  Treat them more like mktime.
* lib/time.in.h (localtime, gmtime):
* m4/gettimeofday.m4 (gl_GETTIMEOFDAY_REPLACE_LOCALTIME):
* m4/time_h.m4 (gl_HEADER_TIME_H_DEFAULTS):
* modules/time (time.h):
Don't worry about about the possibility of localtime and gmtime
being absent; they're present in all C libraries we know about.
* m4/time_h.m4 (gl_HEADER_TIME_H_DEFAULTS):
Don't assume sys_time is present and has been initialized.
Instead, use a hack that should work even if it hasn't been.
Don't use a portcheck for gmtime or localtime; this supports
the hack.
* modules/time (time.h): Substitute GNULIB_GETTIMEOFDAY.