maint: update to latest gnulib
    
    Among other recent changes, this includes a workaround to avoid
    Clang compiler bug https://llvm.org/bugs/show_bug.cgi?id=16404
    having spurious link failures.
    
    * .gnulib: Update.
    * bootstrap: Synchronize to upstream.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>