Exclude quantified constraints code for GHCJS

QuantifiedConstraints isn't available for GHC 8.4 (where our GHCJS
version is still stuck).

We may need to take a drastic decision for GHCJS at some point.

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

@@ -1,16 +1,21 @@
 {-# OPTIONS_GHC -fno-warn-orphans  #-}
 {-# LANGUAGE ConstraintKinds       #-}
+{-# LANGUAGE CPP                   #-}
 {-# LANGUAGE FlexibleContexts      #-}
 {-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE InstanceSigs          #-}
 {-# LANGUAGE KindSignatures        #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE RankNTypes            #-}
 {-# LANGUAGE ScopedTypeVariables   #-}
 {-# LANGUAGE TypeApplications      #-}
 {-# LANGUAGE TypeFamilies          #-}
 {-# LANGUAGE UndecidableInstances  #-}
+
+#if __GLASGOW_HASKELL__ >= 806
+{-# LANGUAGE QuantifiedConstraints  #-}
+#endif
+
 module  Servant.Client.Generic (
     AsClientT,
     genericClient,
@@ -58,6 +63,8 @@ genericClientHoist nt
     m = Proxy :: Proxy m
     api = Proxy :: Proxy (ToServantApi routes)
 
+#if __GLASGOW_HASKELL__ >= 806
+
 type GClientConstraints api m =
   ( GenericServant api (AsClientT m)
   , Client m (ToServantApi api) ~ ToServant api (AsClientT m)
@@ -95,3 +102,5 @@ instance
         fromServant @api @(AsClientT mb) $
         hoistClientMonad @m @(ToServantApi api) @ma @mb Proxy Proxy nat $
         toServant @api @(AsClientT ma) clientA
+
+#endif

servant-server/src/Servant/Server/Generic.hs

@@ -1,6 +1,7 @@
 {-# OPTIONS_GHC -fno-warn-orphans   #-}
 {-# LANGUAGE AllowAmbiguousTypes    #-}
 {-# LANGUAGE ConstraintKinds        #-}
+{-# LANGUAGE CPP                    #-}
 {-# LANGUAGE DataKinds              #-}
 {-# LANGUAGE DefaultSignatures      #-}
 {-# LANGUAGE FlexibleContexts       #-}
@@ -15,6 +16,11 @@
 {-# LANGUAGE TypeFamilies           #-}
 {-# LANGUAGE TypeOperators          #-}
 {-# LANGUAGE UndecidableInstances   #-}
+
+#if __GLASGOW_HASKELL__ >= 806
+{-# LANGUAGE QuantifiedConstraints  #-}
+#endif
+
 -- | @since 0.14.1
 module Servant.Server.Generic (
     AsServerT,
@@ -113,6 +119,8 @@ genericServerT
     -> ToServant routes (AsServerT m)
 genericServerT = toServant
 
+#if __GLASGOW_HASKELL__ >= 806
+
 -- | Set of constraints required to convert to / from vanilla server types.
 type GServerConstraints api m =
   ( ToServant api (AsServerT m) ~ ServerT (ToServantApi api) m
@@ -173,3 +181,5 @@ instance
             toServant server
           servantSrvN :: ServerT (ToServantApi api) n =
             hoistServerWithContext (Proxy @(ToServantApi api)) pctx nat servantSrvM
+
+#endif