Add support for Mac host

[ahumesky]

android_ndk_repository in rules.bzl is currently hard-coded for Linux.

[cpsauer]

Thanks so much @ahumesky!
[Subscribing and will play around more just as soon as this lands.]

[keith]

@ahumesky is this something you're planning on picking up or should someone from the community take it on?

[keith]

❤️

[ahumesky]

It looks like all that was needed was to select the right clang directory, and the host_system_name is no longer used, so that could just be deleted. I can build the example app on both linux and mac now.