Do not warn by default when policy CMP0025 or CMP0047 is not set
These policies are triggered by the use of a particular compiler rather than outdated CMake code in a project. Avoid warning in every project that enables a language by not displaying the policy warning by default. Add variable CMAKE_POLICY_WARNING_CMP<NNNN> to control the warning explicitly; otherwise enable the warning with --debug-output or --trace. This breaks with strict policy convention because it does not provide developers with any warning about the behavior change by default. Existing projects will continue to build without a warning or change in behavior. When a developer changes the minimum required version of CMake in a project to a sufficiently high value (3.0), the project will suddenly get the new compiler id and may break, but at least the breakage comes with a change to the project rather than the version of CMake used to build it. Breaking strict policy convention is worthwhile in this case because very few projects will be affected by the behavior change but every project would have to see the warning if it were enabled by default.
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