Fix Darwin&CPU build

[evelkey]

Build for Darwin platform and CPU-only has been broken for a long time, now it is fixed and should work by simply running --cpu-only or having no GPU and using --blas=mkl for acceleration.

• [x] The below issue is solved as well on Mac machines using clang:

  Python 3.8.11 (default, Sep 23 2021, 14:53:39)
  [Clang 10.0.0 ] :: Intel Corporation on darwin
  Type "help", "copyright", "credits" or "license" for more information.
  Intel(R) Distribution for Python is brought to you by Intel Corporation.
  Please check out: https://software.intel.com/en-us/python-distribution
  >>> import MinkowskiEngine as ME
  Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/x/opt/anaconda3/envs/y/lib/python3.8/site-packages/MinkowskiEngine-0.5.4-py3.8-macosx-10.9-x86_64.egg/MinkowskiEngine/__init__.py", line 53, in <module>
  from MinkowskiEngineBackend._C import (
  ImportError: dlopen(/Users/x/opt/anaconda3/envs/y/lib/python3.8/site-packages/MinkowskiEngine-0.5.4-py3.8-macosx-10.9-x86_64.egg/MinkowskiEngineBackend/_C.cpython-38-darwin.so, 0x0002): symbol not found in flat namespace '__ZNK2at10TensorBase8data_ptrIlEEPT_v'

Tested with both torch versions 1.10 and current master. This PR fixes https://github.com/NVIDIA/MinkowskiEngine/issues/106.

[evelkey]

@chrischoy Can you please review this PR?

[chrischoy]

Thanks for the contribution!