tweaks to mac & debugging

[pmarks]

• give a better error message if we can't find the right mkl package.
• latest MKL on Mac needs a different search path.

[mike-kfed]

thanks for the hint, helped me to get my ndarray-linalg running! I installed Intels MKL using their oneapi installer, my workaround was to do this:

sudo mkdir -p /opt/intel/mkl/lib/
sudo ln -s /opt/intel/oneapi/mkl/latest/lib /opt/intel/mkl/lib/intel64

maybe those oneapi /opt/intel/oneapi/mkl/latest/lib install directories can/should be added too in your patch?

[termoshtt]

This becomes out-dated due to the large rewrite from #81 to #90, and some of problems are resolved.