build: update gnulib submodule to latest

Mainly with build fixes for FreeBSD and OS X.

diff --git a/gnulib b/gnulib
index 49078a780041205fbbab56802033595eb44f854d..551098026bd1e4e10aeccc8dde40539fbc325082 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 49078a780041205fbbab56802033595eb44f854d
+Subproject commit 551098026bd1e4e10aeccc8dde40539fbc325082