Re-instate libvirt_checking_message.
Re-instate libvirt_checking_message.

Older mkmf do not have checking_message, so we implement
our own for compatibility.

Signed-off-by: Chris Lalancette <60b62644009db6b194cc0445b64e9b27bb26433a@redhat.com>

pronix/ruby-libvirt,pronix/ruby-libvirt,pronix/ruby-libvirt