[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
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 A few months ago we reverted a cleanup
 (cac3fb06f4b282eee21159c364c4d08e8fdedce9) to `PosixSource.h` due to
 apparent [[http://haskell.inf.elte.hu/builders/solaris-
 amd64-head/573/10.html|breakage]] on Solaris. We need to sort this out
 since the cleanup also apparently fixes things on AArch64.

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