Fix build with GHC 7.4.2

Author: grayjay

cabal-install/Distribution/Client/Dependency/Modular/Linking.hs

@@ -512,7 +512,7 @@ lgMerge blame lg lg' = do
                           )
 
 varsToConflictSet :: Set (Var QPN) -> ConflictSet QPN
-varsToConflictSet = M.fromSet (const ConflictAll)
+varsToConflictSet = S.foldr (\var -> M.insert var ConflictAll) M.empty
 
 lgConflictSet :: LinkGroup -> Set (Var QPN)
 lgConflictSet lg = S.fromList (map aux (S.toList (lgMembers lg)) ++ lgBlame lg)

cabal-install/Distribution/Client/Dependency/Modular/Preference.hs

@@ -9,7 +9,6 @@ import qualified Data.Traversable as T
 #if !MIN_VERSION_base(4,8,0)
 import Control.Applicative
 #endif
-import qualified Data.Set as S
 import Prelude
 import Control.Monad.Reader
 import Data.Ord
@@ -443,5 +442,5 @@ enforceSingleInstanceRestriction = (`runReader` M.empty) . cata go
           local (M.insert inst qpn) r
         (Nothing, Just qpn') -> do
           -- Not linked, already used. This is an error
-          let cs = M.fromSet (const ConflictAll) (S.fromList [P qpn, P qpn'])
+          let cs = M.fromList [(var, ConflictAll) | var <- [P qpn, P qpn']]
           return $ Fail cs MultipleInstances