Add the ability to add a gmock library target

[clalancette]

We currently have a macro to create an ament_gmock executable target:

ament_cmake/ament_cmake_gmock/cmake/ament_add_gmock.cmake

Line 49 in 0fc4000

macro(ament_add_gmock target)

. However, sometimes it would be nice to make an ament_gmock library target instead; one particular example where this would be useful is in ros2/rviz#540.

The suggestion here is to either add a new macro called something like ament_add_gmock_library(), or to add a LIBRARY option to the existing ament_add_gmock macro.