[GHC] #13792: Allow building using distro GCC on Windows

[GHC]

#13792: Allow building using distro GCC on Windows
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           Reporter:  Phyx-             |             Owner:  (none)
               Type:  feature request   |            Status:  new
           Priority:  normal            |         Milestone:  8.4.1
          Component:  Build System      |           Version:  8.0.1
           Keywords:                    |  Operating System:  Windows
       Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
          Test Case:                    |        Blocked By:
           Blocking:                    |   Related Tickets:
Differential Rev(s):                    |         Wiki Page:
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 Allow the building of GHC on Windows using distro compiler (msys2) instead
 of using a bundled GCC.

 This should make testing new GCCs and binutils easier but also make it
 easier to build the msys2 variant of GHC.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13792>
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