build: update gnulib to latest; portability fixes

diff --git a/gnulib b/gnulib
index d60f3b0c6b0f93a601acd1cfd3923f94ca05abb0..fa6be5b2b297b569d890535b0126054789f8a6fb 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit d60f3b0c6b0f93a601acd1cfd3923f94ca05abb0
+Subproject commit fa6be5b2b297b569d890535b0126054789f8a6fb