build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 91fd934273607a4a395917d5230bd8dd5c866d94..39434c8500eba74bb0c3284a61bc2a8bba633c9f 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 91fd934273607a4a395917d5230bd8dd5c866d94
+Subproject commit 39434c8500eba74bb0c3284a61bc2a8bba633c9f