add DebugLocks build flag

Using the method described in
https://www.fpcomplete.com/blog/2018/05/pinpointing-deadlocks-in-haskell
but my own code to implement it, and with callstacks added.

This work is supported by the NIH-funded NICEMAN (ReproNim TR&D3) project.

Messages.hs

@@ -262,7 +262,7 @@ implicitMessage = whenM (implicitMessages <$> Annex.getState Annex.output)
  - the user.
  -}
 prompt :: Annex a -> Annex a
-prompt a = go =<< Annex.getState Annex.concurrency
+prompt a = debugLocks $ go =<< Annex.getState Annex.concurrency
   where
 	go NonConcurrent = a
 	go (Concurrent {}) = withMessageState $ \s -> do