Refactor fragile build scripts

The old build scripts were fragile; they were not tied to a fixed
naclports revision, and broke from time to time whenever naclports
breaks backward compatibility.

The new build scripts uses git-submodule to tie us to a fixed revision
of naclports (and dosbox).  Ideally we would no longer suffer from
backward incompatible changes.

We also removes much of our build scripts by "properly" reusing
naclports' build scripts.  We used to import naclports' build scripts
and massage them into our build scripts; this approach was complicated
and unmaintainable.  From now on, we will simply be a naclports package.

We also fixes build break due to naclports SDL API changes.