Fix libheif build on macOS and Windows
This PR adds the necessary workarounds to deal with old GCC.
@ivany: can you please try libheif with your M1 before committing? Handbrake reverted the patch that should've fixed this, and I couldn't find any reason in their issue tracker.
Test Plan
Build deps on M1 and Windows (the latter with GCC 7.3)
Formalities Checklist
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