[cmake-developers] Automatically set policies for shipped modules

[Rolf Eike Beer]

I have seen multiple commits basically wrapping Modules/* in 
cmake_policy(PUSH|POP). I neither find that appealing nor do I think 
that this will scale for maintenance.

I suggest that every module included from the CMake installation is 
considered clean for whatever we do and automatically gets a policy 
scope push/pop right from the C++ level. The policies are all set to 
new. This has several advantages:

-we can just use whatever things CMake offers in them
-adding new policies during development would more likely cause things 
to "break" earlier, i.e. if a module is not clean for a new policy we 
may notice this earlier
-processing of the modules themselves should be slightly faster as the 
whole compat code paths are not taken into account right from the start

If a module needs to offer e.g. a function with a specific "old" 
behavior it can still explicitely set this policy to whatever value it 
needs. It does not need to push/pop as that will be done by the C++ code 
anyway.

Opinions?

Eike