Fix ghc-7.8.4 build

Author: Catherine Galkina

servant-client/src/Servant/Client/Generic.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE FlexibleContexts #-}
@@ -6,6 +7,9 @@
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}
+
+#include "overlapping-compat.h"
+
 module Servant.Client.Generic
   ( ClientLike(..)
   , genericMkClientL
@@ -142,7 +146,7 @@ instance (GClientList b acc, GClientList a (ClientList b acc))
   => GClientList (a :<|> b) acc where
   gClientList (a :<|> b) acc = gClientList a (gClientList b acc)
 
-instance {-# OVERLAPPABLE #-} (ClientList client acc ~ (client ': acc))
+instance OVERLAPPABLE_ (ClientList client acc ~ (client ': acc))
   => GClientList client acc where
   gClientList c acc = I c :* acc