Add custom build backend to support build args

[janEbert]

This implements a custom build backend, inspired by the solution used by Pillow.

--build-options aren't actually passed to the build_wheel command for reasons unknown to me. So this patches setuptools.build_meta to take the flags manually.

The setuptools version requirement of 40.8.0 was also adopted from Pillow. It is the version that introduced the build_meta:__legacy__ backend.

This actually fixes what #1690 meant to do and seems fully backward-compatible.

[janEbert]

Ref #1679.

[Quentin-Anthony]

I and others are still facing issues that this PR, coupled with https://github.com/NVIDIA/apex/pull/1680, would resolve. Is there anything I can do (testing, fix any remaining hangups, etc) to get this merged into upstream?

[loadams]

@crcrpar - thoughts on merging this PR?