Pulls in several portability fixes, including the fact that gnulib
now only works on platforms with two's complement signed integers.
Also makes for a smaller delta on the next update (we are waiting
on a license change to unsetenv for the sake of mingw).

* .gnulib: Update to latest.
* bootstrap: Resync from upstream.
* tests/virstringtest.c: Drop use of obsolete probes of integer
properties.
Signed-off-by: NEric Blake <eblake@redhat.com>