cmake install does not support macOS

[dotlambda]

The library extension is hardcoded as .so:

https://github.com/deltachat/deltachat-core-rust/blob/0691aa3d2ce1a7c56d920dd01acdd35f93cf637c/CMakeLists.txt#L9

It should be .dylib on Darwin. I'm not a CMake expert so I don't know how adapting that dynamically is usually achieved.

cc @Simon-Laux @link2xt