If you don't license per machine, what happens with regard to an automated build machine or build server? There is no developer there.

If no development is performed on the build machine and its sole purpose is to generate actual builds of your products, then no you do not need to purchase a separate license for the build machine. However, if a developer uses the build machine for active development and this developer does not have a valid license for the products, then yes, the developer must acquire a license for the product.