We no longer use any of MSVCRT's stat-functions, so there's no need to
stick to a CRT-compatible 'struct stat' either.
Define and use our own POSIX-2013-compatible 'struct stat' with nanosecond-
precision file times.
Note: Due to performance issues when using git variants with different file
time resolutions, this patch does *not* yet enable nanosecond precision in
the Makefile (use 'make USE_NSEC=1').
Signed-off-by: Karsten Blees <blees@dcon.de>