[FEA] Modernize CMake Build Structure

[mdemoret-nv]

Discovered in PR #3367, our CMake build architecture could benefit from moving to a more modern design using targets instead of the current implementation. This is a blocker for migrating to scikit-build. A non-complete list of improvements to make:

• Remove all uses of CMAKE_CXX_FLAGS and CMAKE_CUDA_FLAGS
• Use generator functions for option dependent target_include_directories and target_link_libraries
  • For example, $<$<BOOL:BUILD_CUML_MPI_COMMS>:${MPI_CXX_INCLUDE_PATH}>
• Switch to target based architecture and export necessary includes and linked libraries
• Create a configuration file to install allowing other CMake libraries to use find_package(CUML)
• Use test configuration in cmake (Optional)
• Decrease use of variables that wont be cached or exported (Optional but recommended)

A first pass was done for the linked PR and can be seen here: https://github.com/rapidsai/cuml/blob/6c2ebde695508ebdcab9ea641719d96f2775849b/cpp/CMakeLists.txt (Very much a WIP)

[dantegd]

There is a related issue as well regarding cmake 3.18 features: #2194, linking it so that we close both

[github-actions]

This issue has been marked stale due to no recent activity in the past 30d. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed. This issue will be marked rotten if there is no activity in the next 60d.

[github-actions]

This issue has been labeled inactive-90d due to no recent activity in the past 90 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed.

[dantegd]

PR #3844 addressed this, we can open new issues for further improvements when/as needed.