Fix rejection of non-ambigious options

This GetOpt patch was sent the the libraries list by Eelis van der Weegen
with the explanation:
There is a bug in System.Console.GetOpt causing it to mistakenly reject
options as ambiguous. Example:
  optsDesc = [Option "" ["color", "colour"] (ReqArg id "color") ""]
Output:
  option `--col' is ambiguous; could be one of:
      --color=color, --colour=color  Foreground color
      --color=color, --colour=color  Foreground color
This error is silly, because the two alternatives listed are the same option.

Distribution/GetOpt.hs

@@ -51,7 +51,7 @@ module Distribution.GetOpt (
    -- $example
 ) where
 
-import Data.List ( isPrefixOf, intersperse )
+import Data.List ( isPrefixOf, intersperse, find )
 
 -- |What to do with options following non-options
 data ArgOrder a
@@ -201,7 +201,8 @@ getNext a            rest _        = (NonOpt a,rest)
 longOpt :: String -> [String] -> [OptDescr a] -> (OptKind a,[String])
 longOpt ls rs optDescr = long ads arg rs
    where (opt,arg) = break (=='=') ls
-         getWith p = [ o  | o@(Option _ xs _ _) <- optDescr, x <- xs, opt `p` x ]
+         getWith p = [ o  | o@(Option _ xs _ _) <- optDescr
+                          , find (p opt) xs /= Nothing]
          exact     = getWith (==)
          options   = if null exact then getWith isPrefixOf else exact
          ads       = [ ad | Option _ _ ad _ <- options ]