[REVIEW] Use FetchContent to get jitify and libcudacxx

[harrism]

Following on #5315, for consistency this PR uses FetchContent to get the source of jitify and libcudacxx rather than using git submodules.

Also updates to use the latest jitify branch (cudf_0.15) to fix CUDA 11 testing issues.

Closes #5369.

[kkraus14]

Just need to update these lines to fix the Cython errors: https://github.com/rapidsai/cudf/blob/branch-0.15/python/cudf/setup.py#L47-L49

Can remove the thrust / cub lines and then update the path to the libcudacxx/include.

[kkraus14]

May want to move this above the CUDA toolkit includes as well just in case it ships in the toolkit in the future.

[harrism]

Just need to update these lines to fix the Cython errors: https://github.com/rapidsai/cudf/blob/branch-0.15/python/cudf/setup.py#L47-L49

Can remove the thrust / cub lines and then update the path to the libcudacxx/include.

Thanks. We can't remove the thrust/cub lines or Cython will build using the Toolkit version of thrust/cub while everything else uses the FetchContent version. I will update the libcudacxx line.

[kkraus14]

Thanks. We can't remove the thrust/cub lines or Cython will build using the Toolkit version of thrust/cub while everything else uses the FetchContent version. I will update the libcudacxx line.

There's no thrust / Cub types exposed in libcudf APIs that are plumbed into Cython so the headers are unused currently.

[harrism]

@kkraus14 I was able to remove those but I had to add the following:

"../../cpp/build/_deps/libcudacxx-src/include",
"../../cpp/build/release/_deps/libcudacxx-src/include",

This is really dependent on where the user decides to put the libcudf build directory. Is there a way to set the build directory generically?

EDIT: going with CUDF_ROOT env variable and falling back to cpp/build if it is not set.

[codecov]

Codecov Report

Merging #5398 into branch-0.15 will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           branch-0.15    #5398   +/-   ##
============================================
  Coverage        88.63%   88.63%           
============================================
  Files               57       57           
  Lines            10769    10769           
============================================
  Hits              9545     9545           
  Misses            1224     1224           

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