[GHC] #12624: Un-revert PosixSource.h cleanup

[GHC]

#12624: Un-revert PosixSource.h cleanup
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        Reporter:  bgamari           |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 I am okay with a platform-specific `#ifdef` assuming that it is precise
 (affecting only Oracle Solaris) and we have a build bot to ensure it
 doesn't break.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12624#comment:7>
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