[GHC] #12572: `readdir_r` is deprecated

[GHC]

#12572: `readdir_r` is deprecated
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        Reporter:  erikd             |                Owner:  erikd
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Building GHC      |  Unknown/Multiple
  failed                             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by erikd):

 Since the implementation already has `#if HAVE_READDIR_R` this fix mostly
 involves detecting whether `readdir_r` is deprecated or not and setting
 `HAVE_READDIR_R` appropriately.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12572#comment:2>
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