[GHC] #12487: configure.ac uses wrong triple information.

[GHC]

#12487: configure.ac uses wrong triple information.
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        Reporter:  Phyx-             |                Owner:  Phyx-
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Build System      |              Version:  8.0.1
      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):  Phab:D2452
       Wiki Page:                    |
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Comment (by Tamar Christina <tamar@…>):

 In [changeset:"18f06878ed5d8cb0cf366a876f2bfea29647e5f0/ghc"
 18f06878/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="18f06878ed5d8cb0cf366a876f2bfea29647e5f0"
 Fix configure detection.

 Summary:
 GHC's configure script seems to normalize the values returned from
 config.guess.
 So for Windows it turns x86_64-pc-mingw64 into x86_64-unknown-mingw32.
 These mangled names are stored in the values $BuildPlatform, $HostPlatform
 and $TargetPlatform.

 However further down the file when the comparison is done between the
 stage0
 compiler and the host the normalized versions are not used.
 So when normalization actually changes the triple this check will fail.

 Not sure why it's worked for all this time.. Nor if this is the right fix?
 Does it still work for cross compiling correctly?

 Test Plan: ./configure

 Reviewers: hvr, austin, thomie, bgamari, erikd

 Reviewed By: erikd

 Subscribers: erikd, #ghc_windows_task_force

 Differential Revision: https://phabricator.haskell.org/D2452

 GHC Trac Issues: #12487
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12487#comment:3>
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