Enable CPU-only builds on macOS

[tilmantroester]

This PR enables CPU-only builds on macOS. It in addition to the changes to spconv, this also requires some changes to ccimport and cumm. They are implemented at https://github.com/tilmantroester/ccimport/tree/macos_support and https://github.com/tilmantroester/cumm/tree/macos_cpu_only. Let me know if you want PRs for those as well.

I've tested this on an arm64 mac, with both Apple and conda clang. For development installs (pip install -e), both JIT and non-JIT work. Standard installs require the non-JIT build.

Compiling with OpenMP works but the results are unstable, so I disabled OpenMP on macOS.

tests/test_conv.py passes for the cases where CPU implementations are available (i.e. ConvAlgo.Native but not for example SparseGlobalMaxPool).