bugfix: build.sh cross-compile on NetBSD host

As kindly reported by Nishimura Toru, the fix is quite simple.

cf: https://groups.google.com/d/msg/minix3/v3yejSogDqY/0TZxzpIHOzUJ

Change-Id: Ibdba8847982e93872217a6e736f50d4c90630e7a
vm: make WARNS=5 proof

Change-Id: I737ded223daf04f1c0c85a2e8e6b36c8fdcd07db