Don't follow symlinks when using Dir::Glob with recursive directories

As it turns out, `Dir.glob` doesn't follow directory symlinks, as evidenced by [this gist](https://gist.github.com/leocassarani/a276c017241ce96e0fa6) (note: `lib/rubinius/documentation` is a symlink).

I chose to use `File::Stat.lstat` because, unlike `stat`, it won't follow symlinks, which means `#directory?` will return `false` when dealing with a symlink that points to a directory.

I'm submitting this as a pull request as it's an unfamiliar part of the codebase.