Xcode 12 build compatibility

[pyby]

Issue type

Feature request

Description of the problem

As a developer:

• [x] I would like to build this library with Xcode 12.
• [x] The output build has to support iOS/iPadOS/tvOS 14. See #228.
• [x] Library dependencies are build also with Xcode 12 (Mantle 2.1.2 needed).

Environment information

• Library version: Latest
• iOS version: 14.0
• Device: Any

[pyby]

See https://github.com/SRGSSR/srgletterbox-apple/issues/230#issuecomment-695844133 with an example how to build the library and its dependencies with Xcode 12.