[GHC] #11569: make -j24 test

[GHC]

#11569: make -j24 test
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        Reporter:  mfox        |                Owner:
            Type:  bug         |               Status:  new
        Priority:  normal      |            Milestone:
       Component:  Test Suite  |              Version:  8.1
      Resolution:              |             Keywords:
Operating System:  Linux       |         Architecture:  aarch64
 Type of failure:  Other       |            Test Case:
      Blocked By:              |             Blocking:
 Related Tickets:              |  Differential Rev(s):
       Wiki Page:              |
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Comment (by rwbarton):

 The failure for T3307 is due to this warning printed by the `make` run by
 the test:
 {{{
 make[1]: warning: jobserver unavailable: using -j1.  Add '+' to parent
 make rule.
 }}}
 I suggest just unsetting `MAKEFLAGS` in `mk/test.mk`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11569#comment:5>
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