Add c-source-dirs: field analogous to hs-source-dirs: and include-dirs:

[dankna]

I came across this need in the course of working with a hybrid Haskell-ObjC project; it just seems weird to set include-dirs but not set this, since it happens in my project to be the same directory. Should be an easy feature; I'm going to implement it and keep track of my progress here.