Rough in lp.h

A header for library plugins (of which DAAP will be one example. Since
I'd like to do DLNA later if I get DAAP to work, no reason not to
abstract it.
Add library plugins (generally) to Makefile
Rough in DAAP plugin

It's just a skeleton, for now.
Add daap plugin build rules to Makefile
Add CONFIG_DAAP option to configure

This option requires that libdmapsharing-3.0 be installed and is off by
default.