Add termbox-sys package, statically link termbox

This removes the system dependency on termbox, instead depending on the
vendored package termbox-sys which will build termbox from source and
link it statically.

By convention, *-sys libraries should also try to probe the current
system for the library, using something like pkg-config, before
resorting to building from source.  However, my distribution's termbox
package does not include pkg-config files at the moment, so adding this
(very desirable!) functionality will need to wait for future work.