sys_time: use link-warning

sys_time: use link-warning

Modernize replacement header, to allow another GNULIB_POSIXCHECK
opportunity.

* m4/sys_time_h.m4 (gl_HEADER_SYS_TIME_H_BODY): Split defaults...
(gl_HEADER_SYS_TIME_H_DEFAULTS): ...into new macro.
(gl_SYS_TIME_MODULE_INDICATOR): New macro.
* modules/sys_time (Depends-on): Add warn-on-use.
(Makefile.am): Always build replacement.
(configure.ac): Update substitutions.
* m4/gettimeofday.m4 (gl_FUNC_GETTIMEOFDAY)
(gl_FUNC_GETTIMEOFDAY_CLOBBER): Include defaults, and no longer
bother with SYS_TIME_H.
* modules/gettimeofday (configure.ac): Declare indicator.
* lib/sys_time.in.h (gettimeofday): Warn if gnulib module is not
in use.

Signed-off-by: Eric Blake <ebb9@byu.net>