Added NetBSD support to libpe's Makefile (identical to Linux)

Hi libpe developers,
while packaging libpe for pkgsrc (NetBSD packages) I had to apply this change. It only extends support for NetBSD though, and support for FreeBSD/OpenBSD/DragonflyBSD/etc should be virtually the same as for Linux, so I think it would make sense to gather them all in a fallback case.
Let me know.
Cheers,
-- khorben