Fix detection of StackOfTasks
Fix detection of StackOfTasks

	* configure.ac: stackoftasks -> StackOfTasks.

florent-lamiraux/hpp-doc,laas/hpp-doc,jmirabel/hpp-doc,jmirabel/hpp-doc,florent-lamiraux/hpp-doc,humanoid-path-planner/hpp-doc,laas/hpp-doc,humanoid-path-planner/hpp-doc