Don't set march=native on Apple Silicon

[xhochy]

Fixes #340

There is the alternative between not stetting anything and using march=armv8. From my understanding, they produce exactly the same results for now.

[devnexen]

Seems accurate or maybe using the check_cxx_compiler_flag directive as alternative might fit too.

[xhochy]

From the packager perspective (I package a lot for conda-forge), I really get annoyed when some upstream library puts march=native per default and without a switch in their code. Thus I would avoid setting it unnecessarily as this pins packages to the machine I build them on and make them thus not-redistributable.

For writing a better patch, I would need a Linux-aarch64 setup which I currently don't have access to.

[devnexen]

Good points, the check_cxx_compiler_flag has just the merit to discard every case we might not have thinkg of when not supported.

[daanx]

Thanks so much! Hope we can make the M1 work :-)