Simplify the definition of many build options.

This plugin has been in MPD for three years, and it has never worked.
Enough!

The hardware is obsolete, and the product does not exist anymore on
the Hauppauge web site.  Let's see if anybody complains about the
removal.

Don't require the caller to provide them.

This plugin is horrible code, I mean it.  Last year, I tried hard to
fix it, but I figured would take less time to do a full rewrite.
Given that I don't even have any device that supports RAOP, I can't do
that properly.  After 16 months, nobody volunteered for fixing it.
Hereby, I delete it, because having no RAOP plugin is better than
having this mess.  Sorry.

Consistent naming.

Move the "extern" declarations from output_list.c, for more type
safety.

Remote Audio Output Protocol (RAOP), for Apple devices.