[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 request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.2
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Easy (less than 1  |         Type of failure:
  hour)                              |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 I wanted to do this to turn -fsimple-list-literals on and off for NoFib
 without editing a million makefiles or recompiling GHC, but I bet there
 are other reasons. GHC's validate should probably prompt the user to
 verify they want to continue of the variable is set, and should issue a
 loud warning at the very end indicating what options were set by the
 variable.

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