[GHC] #15404: ghc-8.6 uninstallable on macos due to hard coded libgmp directory

[GHC]

#15404: ghc-8.6 uninstallable on macos due to hard coded libgmp directory
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        Reporter:  tmcdonell         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.2
       Component:  None              |              Version:
      Resolution:                    |             Keywords:
Operating System:  MacOS X           |         Architecture:
 Type of failure:  Installing GHC    |  Unknown/Multiple
  failed                             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 I see what happened here: we used to build for Darwin using the in-tree
 GMP but this logic was never ported to CircleCI.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15404#comment:6>
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