Deprecate compatibility with CMake versions older than 2.8.12

Brad Kingrequested to merge
brad.king/cmake:deprecate-pre-2.8.12 into master

Issue a deprecation warning on calls to cmake_minimum_required or cmake_policy that set policies based on versions older than 2.8.12. Note that the effective policy version includes ...<max> treatment. This is important in combination with !4755 (merged) for install(EXPORT)-generated files.

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