build: avoid double-close bug with pipe2

Based on Coverity's finding on the previous patch, I audited
gnulib's pipe2 code and found that we had the potential for
a subtle double-close bug, unless gnulib guarantees that the
contents of the fd array are unchanged on pipe2() failure.

* .gnulib: Update to latest, for pipe2 fix.

diff --git a/.gnulib b/.gnulib
index cbfd25f0edf5f4b853e8ed2e01a4782ec80faba2..7269b35c8d9be1a6f97906b9e29b8c422b92fc31 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit cbfd25f0edf5f4b853e8ed2e01a4782ec80faba2
+Subproject commit 7269b35c8d9be1a6f97906b9e29b8c422b92fc31