build_one creates target subdirectory instead of silently failing

Fixed my last pull request (it was a bit of a coincidence it even worked in my webapp).

If you have a scss folder /static/scss and you build to /static/css, libsass finds any files in any subfolder in /static/scss but cannot save the built version because /static/css/subfolder did not exist. Now it tries to create any subfolders in the css directory.