Require CMake 3.5 about cmake-init

[scheibel]

Thanks for the pointer.
I created a follow-up issue to discuss the actual desired minimum version of CMake in #114.

Please note, that our use of CMP0063 will not require the use of CMake 3.3 as we implemented an additional policy existence check:

function(set_policy POL VAL)
    if(POLICY ${POL})
        cmake_policy(SET ${POL} ${VAL})
    endif()
endfunction(set_policy)

In the case of older CMake versions, where a policy is not yet defined, this will result in the OLD behavior instead of the parameter value, which should be manageable.