Allow to pick platforms

[therc]

Perhaps you don't use osx or some wheels just can't be built under it...

[evanj]

We used this with both Mac and Linux. We would publish the platform-specific binary wheels somewhere, then generate rules to download those. Cross compiling wheels is tricky, so I didn't bother to figure it out.

We've decided to not use Bazel, so this is basically unmaintained. I am unlikely to have time to look into this, sorry. :(