configure.ac: check 'tv_nsec' field in 'struct stat'

configure.ac: check 'tv_nsec' field in 'struct stat'

Detect 'tv_nsec' field in 'struct stat' and set Makefile variable
NO_NSEC appropriately.

A side-effect of the above detection is that we also determine
whether 'stat.st_mtimespec' is available, so, as a bonus, set the
Makefile variable USE_ST_TIMESPEC, as well.

Signed-off-by: Reuben Hawkins <redacted>
Helped-by: Eric Sunshine <redacted>
Reviewed-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>