conform to XDG spec in the interpreter

Previously, the interpreter stored built files in the $HOME directory of
the user. To conform to the XDG specification this was changed to $XDG_CACHE_HOME

Author: hubertwas

src/Compile/Options.hs

@@ -39,7 +39,7 @@ import Data.Hashable
 import Control.Monad          ( when )
 import qualified System.Info  ( os, arch )
 import System.Environment     ( getArgs )
-import System.Directory       ( doesFileExist, doesDirectoryExist, getHomeDirectory, getTemporaryDirectory )
+import System.Directory       ( doesFileExist, doesDirectoryExist, getHomeDirectory, getTemporaryDirectory, getXdgDirectory, XdgDirectory(XdgCache) )
 import Platform.GetOptions
 import Platform.Config
 import Lib.PPrint
@@ -882,9 +882,8 @@ getKokaBuildDir "" eval
                 then return kkbuild
                 else do -- avoid the tmp directory as it does not always have execute permissions
                         -- tmp <- getTemporaryDirectory
-                        -- instead use `$HOME/.koka` if in the interpreter
-                        home <- getHomeDirectory
-                        return (joinPath home kkbuild)
+                        -- instead use `$XDG_CACHE_HOME/koka` if in the interpreter
+                        getXdgDirectory XdgCache "koka"
       else return kkbuild
 getKokaBuildDir buildDir _ = return buildDir