CMake: Small fixes for boost, assimp and cuda

[M2-TE]

• set CUDA_ARCHITECTURES property with CMake >= 3.23 while still respecting the CMAKE_* var if set
• find boost in CONFIG mode as preferred by CMake (available since Boost 1.70)
• linking rmagine-core against specific assimp::assimp target, as ASSIMP_INCLUDE_DIRS and ASSIMP_LIBRARIES were not defined in my case (Assimp v5.4.3). The latter two are still used as a fallback, should the alias target be missing
• changed cmake message from Components build: to Components being built:, as it is still the configure stage, not the build stage

[M2-TE]

CMake Error in src/rmagine_core/CMakeLists.txt:
Imported target "assimp::assimp" includes non-existent path
"/usr/lib/include"

Say what now?

[amock]

First of all, thanks for the additions; they all seem to be valid. It seems the assimp package in the ubuntu 20 apt repositories contains a hard-coded path to the include directory. The easiest solution would be to just remove the support for Ubuntu 20. However, I have still some ROS 1 projects running under Ubuntu 20 that are not migrated to ROS 2 yet. So I would be happy if we would find a solution so that it still compiles with Ubuntu 20.

[amock]

Currently, I also have no tests for the GPU part of rmagine. So in the meantime we'll just test your CUDA-related cmake additions internally on different machines.

[M2-TE]

I agree that Ubuntu 20 shouldn't be crossed off the list, even if it's due to a library fault. As you yourself said, it is still very widespread for ROS 1. I've therefore changed the assimp linking portion to this:

# link to assimp::assimp when its library and include dir vars are missing
if (DEFINED ASSIMP_INCLUDE_DIRS AND DEFINED ASSIMP_LIBRARIES)
  target_include_directories(rmagine-core PUBLIC ${ASSIMP_INCLUDE_DIRS})
  target_link_libraries(rmagine-core ${ASSIMP_LIBRARIES})
else()
  target_link_libraries(rmagine-core assimp::assimp)
endif()

This will essentially prioritize the linking (include + libs) via CMake vars, but should still always result in the same outcome as with the ALIAS target