Add macOS CI

[liopeer]

What has changed and why?

• still a draft

[JiwaniZakir]

The ifeq ($(OS),Linux) check in the Makefile (around line 173) is likely broken. On Linux systems, the OS environment variable is not set by the OS itself (unlike Windows where it's Windows_NT), so $(OS) will be empty on Ubuntu CI runners. This means PINNED_TORCH_VERSION_PY38, PINNED_TORCH_VERSION_PY312, and related variables will be undefined when running on Linux CI, silently falling back to whatever default behavior the install targets have. The correct approach would be ifeq ($(shell uname -s),Linux) or checking RUNNER_OS (which GitHub Actions sets to "Linux" or "macOS").

Additionally, the test_unit.yml file is missing a newline at the end of file (noted in the diff), which is a minor but easily fixed style issue. It's also worth considering whether the "Setup cmake" step should be gated with if: ${{ matrix.runners == 'ubuntu-latest' }} the same way FFmpeg is — if cmake isn't needed or behaves differently on macOS, this could cause unexpected failures.