Don't invoke target X if output file is "current"

[John Lindwall]

I'm missing a feature from make.  How can I avoid invoking a target if the "output 
file" is "current" (ie newer then the "input file")?

In ant it seems that this kind of dependency checking is left to the task 
implementation.  Do I have to create a custom task every time I want to implement this 
kind of checking?  DependSet does not seem to be what I want.  

Thanks!