* lib/time.in.h (localtime): Declare also if requested by module
'localtime'.
* lib/localtime.c: New file.
* m4/localtime.m4: New file.
* m4/time_h.m4 (gl_HEADER_TIME_H_DEFAULTS): Initialize GNULIB_LOCALTIME.
* modules/time (Makefile.am): Substitute GNULIB_LOCALTIME.
* modules/localtime: New file.
* doc/posix-functions/localtime.texi: Mention the new module.
