[GHC] #8190: mention --show-options in --help

[GHC]

#8190: mention --show-options in --help
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       Reporter:  carter            |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 GHC head now has a --show-options flag, that lists all of the supported
 ghc options.

 maybe --help should mention this flag, and that every option is specified
 in the ghc manual?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8190>
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