maint: update to latest gnulib

Pick up the new syntax-check speedups.

diff --git a/.gnulib b/.gnulib
index 68b6adebef05670a312fb92b05e7bd089d2ed43a..a2a39436b65f329630df4a93ec4e30aeae403c54 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit 68b6adebef05670a312fb92b05e7bd089d2ed43a
+Subproject commit a2a39436b65f329630df4a93ec4e30aeae403c54