Detect and make use of cabal_macros.h

[thumphries]

Packages using cabal CPP macros are not going to parse properly.

I have no idea if this is even possible, but if we see cabal_macros.h lying around, we could try to make use of it.

Alternatively, an inferior string-matching fallback might be okay.

[thumphries]

Interestingly enough, it's only the function-like macros that make CPP crash. Unfortunately those are the most commonly-used CPP macros in Haskell.

It's also worth looking into alternative ExactPrint functions that ignore CPP altogether.