Fix CMakeLists.txt on 14.04.
Fix CMakeLists.txt on 14.04.

* add_custom_target does not accept argument BYPRODUCTS

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