Set macOS deployment target

Without a macOS deployment target, the resulting library does not work on macOS versions lower than it was currently built on. For example, if libslirp was built on macOS 10.15, it would not work on macOS 10.14. This can be achieved by setting the -mmacosx-version-min=[insert macOS ver here] flag, or using the macOS deployment target if your build system has such setting.

I'd be happy to make a merge request for this, but I don't know what should be the target macOS version.