[GHC] #12752: GHC should be able to warn on usafe CPP usage

[GHC]

#12752: GHC should be able to warn on usafe CPP usage
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        Reporter:  erikd             |                Owner:  erikd
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by erikd):

 I suspect this is just a matter of adding a GHC warning flag named
 something like `-Dcpp-undef` and then if that is set, passing `-Wundef` to
 the C pre-processor.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12752#comment:2>
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