[GHC] #11818: Haddock's Sphinx documentation breaks `make install` if doc building is disabled

[GHC]

#11818: Haddock's Sphinx documentation breaks `make install` if doc building is
disabled
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 slyfox reported that `make install` fails on his setup which lacks
 `sphinx`,

 {{{
 /usr/bin/install -c -m 644  libraries/prologue.txt "/var/tmp/portage/dev-
 lang/ghc-9999/image/usr/share/doc/ghc-9999/html/libraries/"
 /usr/bin/install -c -m 755  libraries/gen_contents_index "/var/tmp/portage
 /dev-lang/ghc-9999/image/usr/share/doc/ghc-9999/html/libraries/"
 for i in utils/haddock/doc/haddock; do \
         cp -Rp $i "/var/tmp/portage/dev-
 lang/ghc-9999/image/usr/share/doc/ghc-9999/html"; \
 done
 cp: cannot stat 'utils/haddock/doc/haddock': No such file or directory
 ghc.mk:936: recipe for target 'install_docs' failed
 make[1]: *** [install_docs] Error 1
 Makefile:129: recipe for target 'install' failed
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11818>
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