Potential fix for https://github.com/buildout/buildout/issues/217

On Windows, `sys.prefix` is a site-package directory, i.e. for a
standard installation `C:\Python27`. The code path for `not
allow_site_packages` removed *everything* from `sys.path` that contains
*any* of the the site-packages directories as reported by
 `site.getsitepackages`, which on Windows removes all pathes below
 `sys.prefix` from `sys.path`.