BLD Add DISTDIR variable. ### Description

Adds `DISTDIR` env variable that points to the directory where libraries should be copied to when building shared libraries with recipe.

### Checklists

- [x] Add new / update outdated documentation

# Credits

Based on the hard work of [@dgym](https://github.com/dgym) on
[cpython-emscripten](https://github.com/dgym/cpython-emscripten).

# Credits

Based on the hard work of [@dgym](https://github.com/dgym) on
[cpython-emscripten](https://github.com/dgym/cpython-emscripten).
