How to pass delegates to C functions?

[Jeroen Bollen]

After being downvoted on stackoverflow for no apperant reason, I 
figured I'd give it a shot here.

How do I pass a delegate to an external C function taking a 
function pointer?

If you want some extra rep on StackOverflow you can also answer 
here: 
http://stackoverflow.com/questions/22845175/pass-delegates-to-external-c-functions-in-d