Minor update to previous two changes.
I originally opted to use
CONFIG[ 'target_os' ] as it contained just what I was looking for, but
it's not there in Ruby 1.6 so that was out. What is there in both 1.6
and 1.8 is 'host_os', but on Linux (and possibly others) it can
contain a tuple value ('linux-gnu' instead of just 'linux').
This change splits the host_os field and takes only the first part
so p4conf.rb should guess correctly in more cases (I think!).