maint: update autoconf submodule

* submodules/autoconf: Up to master.
No significant changes in the files we use (m4sugar.m4 and foreach.m4).

diff --git a/submodules/autoconf b/submodules/autoconf
index 26cb0918d38d756b7c2a3f82226551f6facee47e..9e087dd343b67ac58a7f54e08335ac62fc33ed53 160000 (submodule)
--- a/submodules/autoconf
+++ b/submodules/autoconf
@@ -1 +1 @@
-Subproject commit 26cb0918d38d756b7c2a3f82226551f6facee47e
+Subproject commit 9e087dd343b67ac58a7f54e08335ac62fc33ed53