I wonder why I get the error msg "unresolved inclusion" for stddef.h

The background is this question: https://stackoverflow.com/questions/18754434/altera-de2-hardware-timer-usage where I try to solve the entire hw interrupt mechanism that I now divide into smaller problems to pinpoint what it is that makes the program not tick forward the time when I switch from polling to hardware timer.
What do you think is the matter? Does this problem have anything to do with that time is not ticking forward? When I did it with polling it worked:
https://codereview.stackexchange.com/questions/30870/please-review-polling-for-nios-2
Now I want to do it with hw interrupts and I followed the manual but still have some issues that I hope you can help me with?
Search for
stdio.hin your compiler's path. It could possibly be in some kind ofincludeorincfolder. Then try to see if there is astddef.hat the same place.Try to remove the line including
stddef.hand see if the compiler complains aboutstdio.h.If you find both files, then you probably need to setup your build commands to add the path to the
std*.hfiles (possibly with the-Iflag).