Consider changing the MAC OS instruction using `nproc`

[faze-geek]

Description of issue with documention/Way documentation can be improved

I realized this while setting up cppinterop over the weekend.

Kindly go through this memkind/memkind#33

This -->
cmake --build . --target clang clang-repl lld --parallel $(nproc --all)

should be replaced by -->
cmake --build . --target clang clang-repl lld --parallel $(sysctl -n hw.logicalcpu)

And the change needs to be made at multiple places in the documentation.

[anutosh491]

+1

Being a mac user, I myself struggle with this !

[mcbarton]

@faze-geek @anutosh491 I'll make this change. Rather than write out the instructions twice (one for MacOs and one for Linux). I'll create an environment variable ncpus like we do in the cii, and use that.

[vgvassilev]

In theory you can use cmake --build . --target clang clang-repl lld --parallel which will fall back to the default level of job parallelism of your native tool (such as make or ninja). I am not sure but that likely defaults to your number of cpus...