PR bin/43821 describes the inconsistency that in a '.for i' loop, the
condition '.if ${i:M*.c}' works since 2009 while the seemingly
equivalent condition '.if !empty(i:M*.c)' does not access the variable
'i' from the .for loop but instead the global 'i'.
Resolving this situation in a backwards-compatible and non-surprising
way is hard, as make has grown several features during the last 20 years
that interact in various edge cases. For now, document the most obvious
pitfalls.
rillig