only enable SHA backends that configure finds support for

Backend/WORM.hs

@@ -5,7 +5,7 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-module Backend.WORM (backend) where
+module Backend.WORM (backends) where
 
 import Control.Monad.State
 import System.FilePath
@@ -22,6 +22,9 @@ import Content
 import Messages
 import Types
 
+backends :: [Backend Annex]
+backends = [backend]
+
 backend :: Backend Annex
 backend = Backend.File.backend {
 	name = "WORM",