The old approach of including the same source files into
multiple places is less than pretty.  But moving the code
to libply didn't seem right either, since libply is fairly
generic and could in theory get swapped out with a standard
runtime at some point.

Also, exposing libplybootsplash as an external library will
facilitate creating out of tree splash plugins.

We do this so we can disable them from distcheck,
but still build them easily when not doing distcheck.

and eventually key events

create the start of a boot-splash api for managing the
splash screen.  Setup main.c to use the api (no plugin
exists yet, though, so things are pretty hosed atm)