add a mroe flexible way to specify the gtest sources
add a mroe flexible way to specify the gtest sources

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