build: really fix mingw startup

Aargh; commit 8ae5dfd still didn't fix the mingw problem, because
gnulib defined O_NONBLOCK to 0 for just mingw.  I've now fixed
that in gnulib, but we need the latest pipe2 for libvirt to work.

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

diff --git a/.gnulib b/.gnulib
index 9cc991025d6139a3a8f3e0f1570bf39a66f3aafa..4a1579d7560659ef5723325726eda14490a967f6 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit 9cc991025d6139a3a8f3e0f1570bf39a66f3aafa
+Subproject commit 4a1579d7560659ef5723325726eda14490a967f6