Only include transformers-compat for ghc < 8

Since that's the only place where it's needed
and we don't have to fight with versions elsewhere

gf.cabal

@@ -81,12 +81,12 @@ library
     mtl >= 2.2.1 && < 2.3,
     pretty >= 1.1.3 && < 1.2,
     random >= 1.1 && < 1.3,
-    utf8-string >= 1.0.1.1 && < 1.1,
-    -- We need this in order for ghc-7.10 to build
-    transformers-compat >= 0.6.3 && < 0.7
+    utf8-string >= 1.0.1.1 && < 1.1
 
   if impl(ghc<8.0)
     build-depends:
+      -- We need this in order for ghc-7.10 to build
+      transformers-compat >= 0.6.3 && < 0.7,
       fail >= 4.9.0 && < 4.10
 
   hs-source-dirs: src/runtime/haskell

src/compiler/GF/Interactive.hs

@@ -38,8 +38,10 @@ import GF.Server(server)
 #endif
 
 import GF.Command.Messages(welcome)
+#if !(MIN_VERSION_base(4,9,0))
 -- Needed to make it compile on GHC < 8
 import Control.Monad.Trans.Instances ()
+#endif
 
 -- | Run the GF Shell in quiet mode (@gf -run@).
 mainRunGFI :: Options -> [FilePath] -> IO ()