[GHC] #9500: Allow GHC defaults to be modified with an environment variable

[GHC]

#9500: Allow GHC defaults to be modified with an environment variable
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              Reporter:  dfeuer      |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.2
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Easy (less than 1
  Unknown/Multiple                   |  hour)
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by carter):

 I think https://github.com/ghc/nofib/blob/master/mk/ghc-opts.mk is all you
 need to modify to change the flags passed everywhere

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9500#comment:2>
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