Fix -Wall warnings

Setup.hs

@@ -4,7 +4,6 @@ module Main (main) where
 import Data.List
 import Distribution.Simple
 import Distribution.PackageDescription
-import Distribution.PreProcess
 import Distribution.Setup
 import Distribution.Simple.LocalBuildInfo
 import System.Environment
@@ -25,11 +24,11 @@ extractConfigureArgs :: [String] -> ([String], [String])
 extractConfigureArgs = extractPrefixArgs "--configure-option="
 
 extractPrefixArgs :: String -> [String] -> ([String], [String])
-extractPrefixArgs prefix args
+extractPrefixArgs the_prefix args
  = let f [] = ([], [])
        f (x:xs) = case f xs of
                       (wantedArgs, otherArgs) ->
-                          case removePrefix prefix x of
+                          case removePrefix the_prefix x of
                               Just wantedArg ->
                                   (wantedArg:wantedArgs, otherArgs)
                               Nothing ->