[GHC] #8558: Build xhtml and haddock only when `HADDOCK_DOCS=YES`

[GHC]

#8558: Build xhtml and haddock only when `HADDOCK_DOCS=YES`
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              Reporter:  nomeata     |            Owner:
                  Type:  feature     |           Status:  patch
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.6.3
             Component:  Build       |         Keywords:
  System                             |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by nomeata):

 After you bumped this ticket, I looked at it and thought “hey, that’s
 useful”. So I pushed this as branch `wip/T8558` now (I want travis to
 validate it). And only then noticed that I originally reported this :-]

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8558#comment:3>
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