build: update to latest gnulib

Mainly with build fixes for Mac OS X.

diff --git a/gnulib b/gnulib
index 2aea2c723359d9459fd01e7b428f47656c9ce1e9..98aa8776dd2b510b7f777870fb76b17fd88abbef 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 2aea2c723359d9459fd01e7b428f47656c9ce1e9
+Subproject commit 98aa8776dd2b510b7f777870fb76b17fd88abbef