Add osx-arm64 build

[traversaro]

Fix #29 .

The problem was that the macos build was hardcoding the CC and CXX env variable to clang and clang++, overwriting the correct value set by the conda-build scripts that pointed to the cross-compiler (for osx-arm64 arm64-apple-darwin20.0.0-clang++).

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[traversaro]

@conda-forge-admin, please rerender

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[jwillemsen]

Something to report upstream?

[traversaro]

To be honest, I am not familiar enough with ace build system/vanilla makefiles to have a definite answer. Probably it would be great to only define does variables if they are not already defined?