[GHC] #4496: add a configure build option to use system libffi for Linux

[GHC]

#4496: add a configure build option to use system libffi for Linux
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    Reporter:  juhpetersen       |       Owner:              
        Type:  feature request   |      Status:  new         
    Priority:  normal            |   Milestone:  7.6.1       
   Component:  Build System      |     Version:  7.1         
    Keywords:                    |          Os:  Linux       
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
  Difficulty:  Unknown           |    Testcase:              
   Blockedby:                    |    Blocking:              
     Related:                    |  
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Comment(by igloo):

 On the Mac I have `/usr/lib/libffi.dylib` and ` /usr/include/ffi/*.h`.

 However, I can't see any mingw tarballs that seem likely to include it.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4496#comment:6>
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