[GHC] #8725: make DESTDIR=... install broken

[GHC]

#8725: make DESTDIR=... install broken
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        Reporter:  nomeata              |            Owner:
            Type:  bug                  |           Status:  new
        Priority:  high                 |        Milestone:
       Component:  Build System         |          Version:  7.6.3
      Resolution:                       |         Keywords:
Operating System:  Unknown/Multiple     |     Architecture:
 Type of failure:  Building GHC failed  |  Unknown/Multiple
       Test Case:                       |       Difficulty:  Unknown
        Blocking:                       |       Blocked By:
                                        |  Related Tickets:
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Comment (by nomeata):

 Candidate that might have broken that is
 898cb090c8812704448ec4cb1c10d50df4b7d664/ghc by Ian, and most other
 changes to `ghc.mk` are by Ian.

 Ian, since you seem to be the expert on the build system, do you think you
 can fix this easily?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8725#comment:1>
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