Used for zlib and similar minor libraries.

I've also added an osv/ prefix to make sure we don't clash with system headers.

We probably should either adopt this globally or drop it, but I fear we'll run into
problems with the latter approach.

Silences 'Subproject commit -dirty' messages

Style requires manual edititing of loader.cc to run, no automated way
to pick up whole directories in the manifest, and we won't clear the
environment before running the next .so

Suggested by Christoph.