Make it compile on FreeBSD and NetBSD.

off_t on BSD is 64 bit even on 32bit machines, so there are no
special tricks needed to get large file support. In fact, getconf
has no options for large file support and struct stat64 and lstat64()
do not exist on BSD systems. This caused the compilation to fail on
those systems.

Fix this problem by checking for BSD via uname -s in the Makefile. If
uname indicates we're on BSD, then do not use getconf and #define
stat64 and lstat64() to stat and lstat() respectively.