maint: update to latest gnulib

Time to update to new gnulib before a release.

gcc 5.1 introduced a new -Wformat-signedness, and new gnulib now
turns it on by default.  However, it is still rather lame at the
moment, because it warns for enums, even though there is no way
to control the signeness of an enum which does not use any members
that are negative or larger than INT_MAX, and even though such an
enum would always print the same for both %d and %u:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66249

In file included from ../../src/util/virarch.c:26:0:
../../src/util/virarch.c: In function 'virArchFromHost':
../../src/util/virarch.c:180:15: error: format '%d' expects argument of type 'int', but argument 9 has type 'unsigned int' [-Werror=format=]
     VIR_DEBUG("Mapped %s to %d (%s)",

So this patch turns off the new warning as part of enabling all
other new gcc 5.1 warnings that gnulib now enables.

* .gnulib: Update to latest, in part for gcc 5.1 interaction.
* m4/virt-compile-warnings.m4: Ignore -Wformat-signedness, for now.
Signed-off-by: NEric Blake <eblake@redhat.com>

.gnulib @ 875ec93e

-Subproject commit 106a3866d01f9dd57ab4f10dbeb0d5a8db73a9f7
+Subproject commit 875ec93e1501d2d2a8bab1b64fa66b8ceb51dc67

m4/virt-compile-warnings.m4

@@ -59,6 +59,8 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     dontwarn="$dontwarn -Waggregate-return"
     # gcc 4.4.6 complains this is C++ only; gcc 4.7.0 implies this from -Wall
     dontwarn="$dontwarn -Wenum-compare"
+    # gcc 5.1 -Wformat-signedness mishandles enums, not ready for prime time
+    dontwarn="$dontwarn -Wformat-signedness"
 
     # gcc 4.2 treats attribute(format) as an implicit attribute(nonnull),
     # which triggers spurious warnings for our usage