The -no-undefined I added previously caused failures on Mac.
On my Linux and NetBSD tests it didn't notice. But the Macmini
detected it.  The top of error output is clear:

Undefined symbols:
  "isc::asiolink::IOAddress::operator unsigned int() const", referenced
from:
      isc::dhcp::IfaceMgr::send(boost::shared_ptr<isc::dhcp::Pkt4>
const&)in libdhcp___la-iface_mgr.o

Okayed by tomek via jabber.