libiscsi: support old automake

XCode 3.2.6 on 10.6 ships with automake 1.10 which doesn't understand
AM_SILENT_RULES (which was added in automake 1.11). Use the workaround
on
https://www.flameeyes.eu/autotools-mythbuster/automake/silent.html#idp32522960
to conditionally use it.