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: NEric Blake <eblake@redhat.com>

.gnulib @ f39477db

-Subproject commit 875ec93e1501d2d2a8bab1b64fa66b8ceb51dc67
+Subproject commit f39477dba778e99392948dd3dd19ec0d46aee932