[CMake] CMP0005 warning as default

[Dieter Oberkofler]

The warning offers a lot of information on how to disable but is it really
intended to get the following warning by default?

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CMake Warning (dev) at cmake/Definitions.cmake:51 (ADD_DEFINITIONS):
  Policy CMP0005 is not set: Preprocessor definition values are now escaped
  automatically.  Run "cmake --help-policy CMP0005" for policy details.  Use
  the cmake_policy command to set the policy and suppress this warning.
Call Stack (most recent call first):
  CMakeLists.txt:126 (INCLUDE)
This warning is for project developers.  Use -Wno-dev to suppress it.
----------

If I understand the new policies correctly it should rather apply the policy
by default and allow to set it to OLD for compatibility reasons.

-Dieter