autogen: Only do submodule update if we don't have libgsystem

Otherwise we'll fail due to not being a git repository in the
embedded-dependency tarball case.

autogen.sh

@@ -21,8 +21,8 @@ if test ! -f src/libgsystem/README;
 then
   echo "+ Setting up submodules"
   git submodule init
+  git submodule update
 fi
-git submodule update
 
 autoreconf --force --install --verbose