Disable stack support for ghc <7.8

Language/Haskell/GhcMod/Cradle.hs

@@ -79,6 +79,10 @@ cabalCradle wdir = do
 
 stackCradle :: (IOish m, GmOut m) => FilePath -> MaybeT m Cradle
 stackCradle wdir = do
+#if !MIN_VERSION_ghc(7,8,0)
+    -- GHC < 7.8 is not supported by stack
+    mzero
+#endif
     cabalFile <- MaybeT $ liftIO $ findCabalFile wdir
 
     let cabalDir = takeDirectory cabalFile

test/CabalHelperSpec.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 module CabalHelperSpec where
 
 import Control.Arrow
@@ -57,11 +58,13 @@ spec = do
               then forM_ opts (\o -> o `shouldContain` ["-no-user-package-conf","-package-conf", cwd </> "test/data/cabal-project/.cabal-sandbox/"++ghcSandboxPkgDbDir bp])
               else forM_ opts (\o -> o `shouldContain` ["-no-user-package-db","-package-db",cwd </> "test/data/cabal-project/.cabal-sandbox/"++ghcSandboxPkgDbDir bp])
 
+#if !MIN_VERSION_ghc(7,8,0)
         it "handles stack project" $ do
             let tdir = "test/data/stack-project"
             [ghcOpts] <- map gmcGhcOpts . filter ((==ChExeName "new-template-exe") . gmcName) <$> runD' tdir getComponents
             let pkgs = pkgOptions ghcOpts
             sort pkgs `shouldBe` ["base", "bytestring"]
+#endif
 
         it "extracts build dependencies" $ do
             let tdir = "test/data/cabal-project"