* lib/stat-time.h (USE_BIRTHTIME): Remove.

* lib/stat-time.h (USE_BIRTHTIME): Remove.
(get_stat_atime_ns, get_stat_ctime_ns, get_stat_mtime_ns):
(get_stat_birthtime_ns): Do not try to use "spare" fields.
(get_stat_birthtime_ns): Simplify compile-time tests.
(get_stat_birthtime): Change the API to look like
get_stat_mtime etc., except return a negative tv_nsec on error.
* m4/stat-time.m4 (gl_STAT_TIME, gl_STAT_BIRTHTIME):
Don't check for "spare" fields.
(gl_STAT_BIRTHTIME): Don't check for struct stat.st_birthtimespec.tv_sec
or for struct stat.st_birthtime, as these tests aren't used.
* tests/test-stat-time.c (test_birthtime): Adjust to new API.