[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:  patch
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #8439
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Comment (by carter):

 I've made some improvements and simplifications based upon Austin's
 excellent feedback.

 the logic now correctly sets the haskell cpp-flags automatically when the
 selected cpp program is one of
 gcc, clang, or cpphs, and issues a warning if its a different program from
 those three and
  suggest explicitly setting --with-cpp-flags=FLAGS

 https://github.com/cartazio/ghc/compare/cpp_settings

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8683#comment:13>
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