build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index d4cecf1ee37f6b611ef092c9c7fa3761d02ce2c3..639607db00b96f33b0e18afa27188456c6ccad0a 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit d4cecf1ee37f6b611ef092c9c7fa3761d02ce2c3
+Subproject commit 639607db00b96f33b0e18afa27188456c6ccad0a