Log Message:
Add support for DESTDIR

Fix building math/z3 in privileged mode when not building as root:
ocamlfind: Cannot mkdir /usr/pkg/lib/ocaml/site-lib/Z3: Permission denied

This now uses the option -destdir when calling ocamlfind(1). It also
includes a workaround for what might be a bug in ocamlfind(1), where it
also wants to update ld.conf in spite of specifying -destdir.

Tested on NetBSD/amd64.

ok dholland@