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

[GHC]

#12487: configure.ac uses wrong triple information.
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           Reporter:  Phyx-          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Build System   |           Version:  8.0.1
           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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 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..

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