[GHC] #657: add WARNING pragma

[GHC]

#657: add WARNING pragma
----------------------+-----------------------------------------------------
 Reporter:  ijones    |          Owner:             
     Type:  task      |         Status:  new        
 Priority:  normal    |      Milestone:  6.6.1      
Component:  Compiler  |        Version:  6.4.1      
 Severity:  normal    |     Resolution:             
 Keywords:            |     Difficulty:  Easy (1 hr)
 Testcase:  N/A       |   Architecture:  Unknown    
       Os:  Unknown   |  
----------------------+-----------------------------------------------------
Changes (by igloo):

  * milestone:  => 6.6.1
  * testcase:  => N/A

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/657>
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