Install autoconf as a submodule to get m4sugar.

* .gitmodules: Add submodules/autoconf.
	* data/m4sugar/foreach.m4, data/m4sugar/m4sugar.m4: Now links into
	submodules/autoconf.
(cherry picked from commit 6c63b895fb)

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "gnulib"]
 	path = gnulib
 	url = git://git.savannah.gnu.org/gnulib.git
+[submodule "submodules/autoconf"]
+	path = submodules/autoconf
+	url = git://git.sv.gnu.org/autoconf.git