[GHC] #10223: Cleanup `mk/build.mk.sample`

[GHC]

#10223: Cleanup `mk/build.mk.sample`
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              Reporter:  thomie      |             Owner:  thomie
                  Type:  task        |            Status:  new
              Priority:  normal      |         Milestone:  7.12.1
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10223>
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