Fix path to include used from multiple locations

When this code is instantiated from zc_install it sometimes triggers a path error. Using autodetection avoids that.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/zencart/zc-v1-series/246)
<!-- Reviewable:end -->