improve support for system or ROS pre-installed OpenVDB

Thank you for this excellent project. I'm interested in integrating VDBFusion into RTAB-Map https://github.com/introlab/rtabmap/issues/1286. Since RTAB-Map already has too many optional dependencies, and we only need the C++ API, I prefer to use system or ROS pre-installed OpenVDB. The following changes allow CMake to find them. Compilation has been tested on Ubuntu 20.04 (using system OpenVDB 6.2.1) and Ubuntu 22.04 (using OpenVDB 10.0.1 from ros-humble-openvdb-vendor). Note that these changes do not work with system OpenVDB 8.1.0 in Ubuntu 22.04.