[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 request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Build System      |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by hvr):

 > I suggest to not builds haddock and its dependencies (xhtml, anything
 else?) if HADDOCK_DOCS=YES is specified.

 There's a negation-typo in there, right?

 Your proposal looks good to me.

 However, wrt to the package-tag suggestion: I think the tag column in
 `packages` makes the build-system a bit more complicated just to save
 cloning/syncing a few small repositories; and as a side-effect of #8545 I
 was planing to get rid of the package-tags altogether.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8558#comment:1>
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