build: avoid strerror_r pitfalls

In particular, this guarantees that virStrerror always gives a
useful result.

* .gnulib: Update to latest, for (lots of) strerror_r fixes.

.gnulib @ 9d196fad

-Subproject commit 2c25c9ebe8db1415bfde25f0a451767332c8cf59
+Subproject commit 9d196fad055a448c5732a8e950cc044b353d2615