* MODULES.html.sh (File system functions): Add stat-time.

* MODULES.html.sh (File system functions): Add stat-time.
* lib/stat-time.h: New file.
* lib/timespec.h (ST_TIME_CMP_NS, ST_TIME_CMP, ATIME_CMP, CTIME_CMP):
(MTIME_CMP, TIMESPEC_NS): Remove.  Now done by stat-time.h,
in a different way.
(timespec_cmp): New function.
* m4/st_mtim.m4: Remove.  Superseded by...
* m4/stat-time.m4: New file.
* m4/timespec.m4 (gl_TIMESPEC): Require AC_C_INLINE.
Do not invoke AC_STRUCT_ST_MTIM_NSEC; no longer needed.
* modules/stat-time: New file.
* modules/timespec (Files): Remove m4/st_mtim.m4; this
is now done in a different way, by the stat-time module.