In FreeBSD 8.4 this member of struct stat is named st_mtimespec,
instead of st_mtim. In the early days of FreeBSD 9 (after the
branch but before 9.0-RELEASE), it was renamed to st_mtim in src
rev r205792 for POSIX 2008 compliance. This change was never
merged back to 8-STABLE. There was no corresponding bump of
__FreeBSD_version, but the change happened between 900010 and 900011.
Use the old name when building on FreeBSD 8.4.