Rework libcudf CMakeLists.txt to export targets for CPM

[trxcllnt]

These changes rework libcudf's CMakeLists.txt files to export targets for inclusion by other CMake projects. I took inspiration from RMM's CMakeLists.txt. However I'm not a CMake expert, and likely missed/messed up a few things.

I also fixed those annoying -Wstrict-prototypes warnings we see compiling Cython files.

These changes are adequate to build libcudf as a dependency in another CMake project with CPM, for example:

function(find_and_configure_cudf VERSION)

    include(get_cpm)

    CPMAddPackage(NAME  cudf
        VERSION         ${VERSION}
        GIT_REPOSITORY  https://github.com/trxcllnt/cudf.git
        # Can also use a local path to your repo clone for testing
        # GIT_REPOSITORY  /home/ptaylor/dev/rapids/cudf
        GIT_TAG         fix/cmake-exports
        GIT_SHALLOW     TRUE
        SOURCE_SUBDIR   cpp
        OPTIONS         "BUILD_TESTS OFF"
                        "BUILD_BENCHMARKS OFF"
                        "ARROW_STATIC_LIB ON"
                        "JITIFY_USE_CACHE ON"
                        "CUDA_STATIC_RUNTIME ON"
                        "DISABLE_DEPRECATION_WARNING ON"
                        # control where generated JIT headers get placed
                        # so we don't cache-bust ccache every clean build
                        "CUDF_GENERATED_INCLUDE_DIR ${CPM_SOURCE_CACHE}/cudf-build"
    )
endfunction()

find_and_configure_cudf(${CUDF_VERSION})

add_target(${PROJECT_NAME} SHARED "${src_files}")
target_link_libraries(${PROJECT_NAME} rmm::rmm cudf::cudf)

[trxcllnt]

rerun tests

[kkraus14]

rerun tests

[kkraus14]

rerun tests

[kkraus14]

rerun tests

[kkraus14]

@gpucibot merge

[ttnghia]

This PR is sooooo great! Thank you!!!

There is one more thing that I think we can improve it further. This PR still manually includes a list of .cu/.cpp files for the library. It would be much better if we can automate that by cmake. I'm not sure if we can do something similar to this: https://stackoverflow.com/questions/3201154/automatically-add-all-files-in-a-folder-to-a-target-using-cmake

[kkraus14]

This PR is sooooo great! Thank you!!!

There is one more thing that I think we can improve it further. This PR still manually includes a list of .cu/.cpp files for the library. It would be much better if we can automate that by cmake. I'm not sure if we can do something similar to this: https://stackoverflow.com/questions/3201154/automatically-add-all-files-in-a-folder-to-a-target-using-cmake

If we did this then CMake wouldn't be able to detect when changes are made, so CMake configuration would need to be run on every incremental build which is painful. We considered this and decided against doing this.