pack native-library packages as 'binary instead of 'built

Otherwise, a distribution and/or installation ends up with two copies
of the native library. This change is needed because
http://pkgs.racket-lang.org/ now has a "source" variant of each
native-library package.

More generally, use the `distribution-preference` value in a
package's "info.rkt", where the default is 'binary for a
native-library package because it has only "info.rkt" sources.