[builds] Revise DLG update.

autogen.sh

diff --git a/autogen.sh b/autogen.sh
index ff5e46f..649c644 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -190,8 +190,7 @@
     :
   else
     echo "Checking out submodule in \`subprojects/dlg':"
-    git submodule init
-    git submodule update
+    git submodule update --init
   fi
 
   copy_submodule_files

builds/toplevel.mk

diff --git a/builds/toplevel.mk b/builds/toplevel.mk
index 8d5063e..ce7cc9c 100644
--- a/builds/toplevel.mk
+++ b/builds/toplevel.mk
@@ -170,8 +170,7 @@
 
 check_out_submodule:
 	$(info Checking out submodule in `subprojects/dlg')
-	git --git-dir=$(TOP_DIR) submodule init
-	git --git-dir=$(TOP_DIR) submodule update
+	git -C $(TOP_DIR) submodule update --init
 
 copy_submodule:
 	$(info Copying files from `subprojects/dlg' to `src/dlg' and `include/dlg')