Only output the elapsed time at verbose level, so that we don't see it during make...

Only output the elapsed time at verbose level, so that we don't see it during make check (this value changes in RL from run to run, of course, and cannot be strictly checked). Lionel, just increase the verbosity of the processes to see it again

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@4145 48e7efb5-ca39-0410-a469-dd3cf9ba447f