Not only that. In the above mentioned MXE project, the $(...) functions also play a vital role on the "second stage": They create a huge, automatic set of rules

Not only that. In the above mentioned MXE project, the $(...) functions also play a vital role on the "second stage": They create a huge, automatic set of rules and actions, based on a higher-level description that is given for each package via a bunch Make variables (some of which are multiline and contain actual commands for that package).