Hi,
Do you need to use PCL 1.8 under ROS? If not, just not "install" PCL1.8, just build it. Then for your project using PCL1.8, set cmake -DPCL_DIR="path/to/pcl18/build" to the build directory of PCL 1.8 to find the correct PCLConfig.cmake.
If you need PCL 1.8 under ROS, then the best is to remove 1.7 and rebuild all binary packages depending on pcl from source (like
pcl_conversions and pcl_ros). To make rtabmap rebuild correctly without rebuilding g2o and GTSAM, disable
-march=native set by default in PCL 1.8: comment this
line before building 1.8. When rebuilding rtabmap, you can cleanup its build directory to make sure cmake is correctly updating PCL paths to 1.8.
cheers,
Mathieu