Fixed accessing subsecond timestamp values of struct stat on POSIX.1-2008.09 / X/Open 7 systems

According to stat (2) manual http://man7.org/linux/man-pages/man2/stat.2.html#NOTES

This is the fix to the problem already encountered on some Arch Linux distribution (https://groups.google.com/forum/#!topic/ocaml-core/5XB2ds7cYzQ) (I also have Arch). It's  based on the stackoverflow question http://stackoverflow.com/questions/19802098/accessing-subsecond-timestamp-values-of-struct-stat-in-a-portable-way.