build: update to latest gnulib

Pick up an errno adjustment in xstrtol() that fixes
a spurious test failure on Darwin 14.0.0.
Also update copyright year to 2015 avoiding a syntax-check failure.

diff --git a/gnulib b/gnulib
index 7177237ead4c6080705129e6ec3c3a9e9eef2f68..b9bfe78424b871f5b92e5ee9e7d21ef951a6801d 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 7177237ead4c6080705129e6ec3c3a9e9eef2f68
+Subproject commit b9bfe78424b871f5b92e5ee9e7d21ef951a6801d