[GHC] #12619: Allow users guide to be built independently from GHC

[GHC]

#12619: Allow users guide to be built independently from GHC
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        Reporter:  bgamari           |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by thomie):

 The reason we don't build `mkUserGuideParts` with stage0 currently is that
 it depends on the `ghc` library. `Main.hs` import `DynFlags`, although
 currently only to get to `glasgowExtsFlags`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12619#comment:5>
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