make: warn/error on expanding undefined vars?

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

Is there a way to make make error out (or at least emit a warning) if an 
undefined variable is expanded (where, of course, defined() and maybe 
empty() and :U don't count as an expansion)?

It took me hours yesterday to find out that a weird problem stemmed from an 
undefined variable. And it was not a spelling error as usual, but me not 
noticing the the assignment was resinding inside a conditional that was false.

I'm afraid the answer is no because it's difficult to pin down what counts 
as an expansion (i.e., assignment to another variable).