[GHC] #8683: add "cpp program" and "cpp program options" to settings file

[GHC]

#8683: add "cpp program" and "cpp program options" to settings file
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       Reporter:  carter            |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:  7.8.1
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8683>
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