maint: update to latest gnulib

Incorporates fixes for cross-compiling to mingw on rawhide, where
gcc 5.1 changes detection of how to properly determine PRIdMAX.
Also picks up some improvements for compilation on Mac OS X.

* .gnulib: Update to latest, for at least mingw.

Signed-off-by: Eric Blake <eblake@redhat.com>

diff --git a/.gnulib b/.gnulib
index 875ec93e1501d2d2a8bab1b64fa66b8ceb51dc67..f39477dba778e99392948dd3dd19ec0d46aee932 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit 875ec93e1501d2d2a8bab1b64fa66b8ceb51dc67
+Subproject commit f39477dba778e99392948dd3dd19ec0d46aee932