fix build with old ghc that used old-time type

Logs/Unused.hs

@@ -15,6 +15,8 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
+{-# LANGUAGE CPP #-}
+
 module Logs.Unused (
 	UnusedMap,
 	updateUnusedLog,
@@ -90,9 +92,15 @@ readUnusedMap :: FilePath -> Annex UnusedMap
 readUnusedMap = log2map <$$> readUnusedLog
 
 dateUnusedLog :: FilePath -> Annex (Maybe UTCTime)
+#if MIN_VERSION_directory(1,2,0)
 dateUnusedLog prefix = do
 	f <- fromRepo $ gitAnnexUnusedLog prefix
 	liftIO $ catchMaybeIO $ getModificationTime f
+#else
+#warning foo
+-- old ghc's getModificationTime returned a ClockTime
+dateUnusedLog _prefix = Nothing
+#endif
 
 {- Set of unused keys. This is cached for speed. -}
 unusedKeys :: Annex (S.Set Key)