Modernize cmake config template

[lesc-hexagon]

The helper strings was not updated for 14 year and the mentioned Targets do not seem to exist.

Update to use modern style.

[larshg]

With my current test, PCL_INCLUDE_DIRS and PCL_LIBRARY_DIRS are populated with:

[CMake] PCL_INCLUDE_DIRS are C:/dev/pcl/out/install/x64-Release/include/pcl-1.14;C:/dev/vcpkg/installed/x64-windows/include/eigen3
[CMake] PCL_LIBRARY_DIRS are C:/dev/pcl/out/install/x64-Release/lib

So maybe we can keep them as optional and indeed add the modern way as the primary approach.

The values should be populated from the CONFIG script, at least with the root directory of PCL?

# check whether PCLConfig.cmake is found into a PCL installation or in a build tree
if(EXISTS "${PCL_ROOT}/include/pcl-${PCL_VERSION_MAJOR}.${PCL_VERSION_MINOR}/pcl/pcl_config.h")
  # Found a PCL installation
  # pcl_message("Found a PCL installation")
  set(PCL_CONF_INCLUDE_DIR "${PCL_ROOT}/include/pcl-${PCL_VERSION_MAJOR}.${PCL_VERSION_MINOR}")
  set(PCL_LIBRARY_DIRS "${PCL_ROOT}/lib")
elseif(EXISTS "${PCL_ROOT}/include/pcl/pcl_config.h")
  # Found a non-standard (likely ANDROID) PCL installation
  # pcl_message("Found a PCL installation")
  set(PCL_CONF_INCLUDE_DIR "${PCL_ROOT}/include")
  set(PCL_LIBRARY_DIRS "${PCL_ROOT}/lib")
elseif(EXISTS "${PCL_DIR}/include/pcl/pcl_config.h")
  # Found PCLConfig.cmake in a build tree of PCL
  # pcl_message("PCL found into a build tree.")
  set(PCL_CONF_INCLUDE_DIR "${PCL_DIR}/include") # for pcl_config.h
  set(PCL_LIBRARY_DIRS "${PCL_DIR}/lib")
else()
  pcl_report_not_found("PCL can not be found on this machine")
endif()

set(PCL_INCLUDE_DIRS "${PCL_CONF_INCLUDE_DIR}")

[lesc-hexagon]

With my current test, PCL_INCLUDE_DIRS and PCL_LIBRARY_DIRS are populated with:

You are probably right, my description is probably false. I think we tried to link against the Components specific Variables. Which do not exist for all Components or something.
We probably got confused as the doc said to search for components specifically and then link to a global target.
None the less I think the new way linking against the target would be preferable. But I just thought I propose the update we found working for us. I have not invested much time to understand you full cmake.