makefile: streamline variable checks

[doshitan]

Changes

Reduce boilerplate for commonly used environment variables. And generally rather than more imperatively checking variables, having a target to do the check and moving them to the prerequisite list feels "cleaner".

TODO:

• Don't love require- prefix, was thinking maybe just var-%? Or more verbose check-defined-%/assert-set-%
• Moving to a target instead of inline function call does lose the context in the "error" message of which target it's coming from (though generally should be obvious).

Testing

Run some make commands.

TODO: screenshots