add "cpp program" and "cpp program options" to settings file
so theres a pretty clear consensus that we need to move away from using our c compiler for CPP, so should we add that info to the settings file?
I propose adding the following fields to the settings file:
"use c compiler cpp", which can have either "YES" or "NO". If "YES", the settings file indicated C compiler is used unless the c compiler selection is overrided by ghc at runtime via -pgmc or -pgmp(p?) if "NO", then the "cpp program" and "cpp program options" fields would be used, unless overridden by -pgmp
there'd be some associated patched in the compiler/main/DriverPipeline.hs too i think, but relatively minimal.
this would make it much easier to distributed a GHC with its own CPP tool that works