add a CPP program field to Settings file (decouple CPP program and C compiler choice)
the tight coupling in a vanilla GHC between the choice of c compiler and CPP program (for the normal user, with or without cabal-install managing the build) is a huge source of support problems for GHC on OS X right now. Additionally, even if everyone started having cpphs or something similar installed on their computers tomorrow, we have no way of bundling it with GHC so that it will be called by default for any Haskell CPP needs.
accordingly, maybe it'd be worth adding a cpp program field to settings? i'm aware of -pgmP and how cabal can use it (or I can), but normal ghc packages and naive end users don't twiddle that.
also its been said a number of times recently that GHC should have its own preprocessor, and if so, we need to bake in machinery to be able to distribute a program distinct from the C compiler for that role