added comment to list_append_unique to justify its use over vanilla cmake
added comment to list_append_unique to justify its use over vanilla cmake

thaddeustaylor/catkin,beni55/catkin,asmodehn/catkin,scpeters/catkin,meyerj/catkin,meyerj/catkin,asmodehn/catkin,beni55/catkin,shyamalschandra/catkin,harmishhk/catkin,thaddeustaylor/catkin,wkentaro/catkin,wkentaro/catkin,jack-oquin/catkin,beni55/catkin,wkentaro/catkin,harmishhk/catkin,thaddeustaylor/catkin,asmodehn/catkin,jack-oquin/catkin,harmishhk/catkin,ros/catkin,meyerj/catkin,scpeters/catkin,ros/catkin,ros/catkin,jack-oquin/catkin,scpeters/catkin,shyamalschandra/catkin,shyamalschandra/catkin