Handle case with multiple invocations (avoid pasting ${CMAKE_THREAD_LIBS_INIT} multiple times).
Also check if target 'gmock' already exists before building GMock to avoid failures if GMock was already
find_packaged() from another subproject.
FIXES=https://github.com/googlecartographer/cartographer_ros/issues/776
- Pin versions of Ceres and protobuf (Fixes#341).
- Require Lua >= 5.2 (Fixes#629).
- No longer run ceres tests in CI. We implicitly test Ceres through Cartographer's tests and this saves ~4 minutes on CI.