build: update gnulib submodule to latest (quoting change)

This pulls in changes to the quote and quotearg modules that
render quoted strings in diagnostics.  `Old' vs 'new'.

diff --git a/gnulib b/gnulib
index dc6246c..2cefd73 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit dc6246c5191f6228c7f3a33f56dc81c226868a99
+Subproject commit 2cefd738d1d50d61b5a14d145143697f8e6ef394