#2 macOS: Building module fails

[Joshua Haberman]

I ran into two separate issues when trying to use setuptools-zig on macOS:

1. The compile fails with undefined symbols unless you pass "-target native-macos", see: https://github.com/ziglang/zig/issues/8180#issuecomment-797846607
2. The output file extension from "zig build-lib" is ".dylib", not ".so", so the setup step fails unless you manually rename .dylib -> .so.