maint: revert "build: update gnulib submodule to latest"

Don't update to latest from gnulib just yet.
This reverts commit 173f4d9bd628fb771eaeca3d2fef97dcd71fedb8.
In particular, manywarnings is too aggressive.

diff --git a/gnulib b/gnulib
index 3e07d5d..17febe7 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 3e07d5d60bee7d0e6fc0d79e974ce5a8dc7dea61
+Subproject commit 17febe7763c9ef38ed1ced5f08f022168e198929