maint: update to latest gnulib
    
    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: Eric Blake <eblake@redhat.com>