Do not add_library() gmock and gtest if targets already exist (#927)
Do not add_library() gmock and gtest if targets already exist (#927)

* Do not add_library() gmock and gmock_main() if already existing

Fixes build problems with Google Cartographer (https://github.com/googlecartographer/cartographer_ros/issues/776), which provides its own `FindGMock.cmake`.

* Also check if gtest exists before calling add_library()

ros/catkin,ros/catkin,ros/catkin