Merge changes from gnulib for file system sub-second time stamps.

* configure.ac: Remove checks for struct stat.st_spare1, struct
stat.st_atim.tv_nsec, struct stat.st_atimespec.tv_nsec, struct
stat.st_atimensec, as gnulib now does this for us.
Similarly for LIB_CLOCK_GETTIME.
* gnulib.modules: Add stat-time.
* lib/.cvsignore: Add stat-time.h.
* src/common.h: Include stat-time.h.
(timespec_lt): Remove.  All callers changed to use timespec_cmp.
(get_stat_atime, get_stat_ctime, get_stat_mtime):
(set_stat_atime, set_stat_ctime, set_stat_mtime):
Remove; now defined by stat-time.h.