Don't try to elevate user rights if impossible

On Windows, if the user doesn't have administrative rights, we have
to  query the system policy for UAC. If UAC is not enabled, it is
obviously not possible to use UAC to elevate the user's rights.

Change-Id: Ieccf112efb50a6426277b32516ab58ca467a364e
Reviewed-by: Sascha Cunz <sascha.cunz@gmail.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>