Build: Fix python site-packages path

During the build, your idea for getting the directory right above `site-packages` is

`````python
import sys; sys.stdout.write(sys.path[2])
`````

On most systems, `sys.path` is completely filled with all sorts of modules in all sorts of terrible locations (e.g. `$HOME/.local`, packages imported by [`site`](https://docs.python.org/2/library/site.html), etc.).

My suggestion on how to do it properly is only half-serious, but it adheres completely to the description in  the `site` module docs on how this path should be constructed.