[GHC] #8439: add a CPP program field to Settings file (decouple CPP program and C compiler choice)

[GHC]

#8439: add a CPP program field to Settings file (decouple CPP program and C
compiler choice)
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       Reporter:  carter            |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:  7.10.1
      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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 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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8439>
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