Alternately, just pop up a confirmation dialog stating "I hereby confirm that I am requesting of my own volition to download PNGOUT to aid the functionality of this program," and then download it automatically.
The license clearly specifies that "reusing" is itself not allowed without permission, so that doesn't change anything...his program still uses pngout without permission, and it's his (the programmmer's) use that is at issue here, not the end user.
Also, I think that having it finish quicker is >> correctly indicated timeline.