Simplify the finding of cpp_region library

As setuptools builds `MODULE` instead of `SHARED` libraries, cpp_region will have the name `libcpp_region.so` (not `libcpp_region.dylib` longer). So now it's possible we separate its finding into *nix and windows system.
It looks like you remove the `#if` clause and don't add a new one so the `#elif` below doesn't have a corresponding `#if`