Configure cpp in ghc settings rather than hardcode
Currently, we're configuring the haskell preprocessor and haskell preprocessor flags into the ghc settings file, and ghc uses that configuration to invoke that program.
Unlike the haskell preprocessor, the c preprocessor, used to preprocess Cmm files, is hardcoded in GHC.SysTools.Cpp
as cc -E
.
For the sake of uniformity, we should also configure the C preprocessor into settings, and have GHC trust and use that configuration.
CC @bgamari