真正的控制语句是`// +build !windows`
 原文是这样说的
>A build constraint is a line comment beginning with the directive +build that lists the conditions under which a file should be included in the package.
Constraints may appear in any kind of source file (not just Go), but they must appear near the top of the file, preceded only by blank lines and other line comments.

[这里有更多说明](http://golang.org/pkg/go/build/)
否则对非windows系统的源码，我需要写至少三个一模一样的文件(linux freebsd darwin)
`*_notwin`这个文件名只是方便识别。根据文件名你一下子就猜到了这个文件的作用对吗？