Build with the git-lfs library when available

Otherwise use the vendored copy as before.

The library is in Debian testing but not stable. Once it reaches
stable, the vendored copy can be removed.

Did not add it to debian/control because IIRC that's used to build
git-annex on stable too, possibly. However, the Debian maintainer will
probably want to make the package depend on libghc-git-lfs-dev.

This commit was sponsored by Ilya Shlyakhter on Patreon.

Remote/GitLFS.hs

@@ -6,6 +6,7 @@
  -}
 
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE CPP #-}
 
 module Remote.GitLFS (remote, gen, configKnownUrl) where
 
@@ -39,9 +40,14 @@ import Utility.SshHost
 import Utility.Url
 import Logs.Remote
 import Logs.RemoteState
-import qualified Utility.GitLFS as LFS
 import qualified Git.Config
 
+#ifdef WITH_GIT_LFS
+import qualified Network.GitLFS as LFS
+#else
+import qualified Utility.GitLFS as LFS
+#endif
+
 import Control.Concurrent.STM
 import Data.String
 import Network.HTTP.Types