[GHC] #13111: maintainer-clean should clean build.mk

[GHC]

#13111: maintainer-clean should clean build.mk
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        Reporter:  bgamari           |                Owner:
            Type:  task              |               Status:  patch
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Build System      |              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):  Phab:D2963
       Wiki Page:                    |
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Comment (by bgamari):

 There were objections to changing this behavior.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13111#comment:2>
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