support os_type as variables of conditional compilation

[hackwaly]

Feature Request

Summary

Add windows, linux, macos (All OS(s) we supported) ... as variables of conditional compilation.

Motivation

To support authoring x/path package, we need to know os_type at compile-time and/or runtime.

Detailed Description

It's better to know os_type at compile-time. So DCE can works well.

Related Issues

Checklist

• [ ] I have added all necessary details to make it easy for the maintainers to understand my request.