This was noticed when trying to do distcheck build on non-Linux.
(Our non-Linux automated builder disappeared.)
I tested this and okayed by muks on jabber.

These files are to allow the authors of hooks callouts to compile
their code.  At the moment only the DHCP files are installed; the
DNS files will be added once hooks have been added to the DNS servers.

... as this class is going to be used either for V6 or V4 now.

Having duid.{h, cc} in libdhcp++ causes dependency between two libraries.
Due to this dependency multi-threaded build fails because libs are created
by different threads simultaneously and race condition occurs. Libraries
should be moved to separate folders and have separate makefiles. This
complies with BIND10 structure and will make this issue goe away.

Also add missing library into the makefile - this showed its
absence on OS/X.