[Haskell-cafe] #include path

Hi, I'm trying to compile some code using Cabal. One of the files has a CPP directive #include "undefined.h" The file undefined.h is in the same directory as the file with the directive. If I use the full path name, cabal can compile it. However, if I use the relative path, it complains about not being able to find the file. I tried adding it to extra-source-files, but that didn't help. I see that Hackage packages like Agda use this trick, so I know it's possible. How can I get these paths worked out correctly? There isn't much comment in the Cabal docs. Thanks, Sean