use the standard System.FilePath module instead of our own broken file path manipulation functions

src/GF/Compile/MkConcrete.hs

@@ -36,6 +36,7 @@ import GF.System.Arch
 import GF.UseGrammar.Treebank
 
 import System.Directory
+import System.FilePath
 import Data.Char
 import Control.Monad
 import Data.List
@@ -111,7 +112,7 @@ mkConcrete :: Parser -> Morpho -> FilePath -> IO ()
 mkConcrete parser morpho file = do
   src <- appIOE (getSourceModule noOptions file) >>= err error return
   let (src',msgs) = mkModule parser morpho src
-  let out = suffixFile "gf" $ justModuleName file
+  let out = addExtension (justModuleName file) "gf"
   writeFile out $ "-- File generated by GF from " ++ file
   appendFile out "\n"
   appendFile out (prModule src')