CMake: bump minimum required version

[olafmandel]

This fixes the following deprecation warning generated by newer CMake versions:

CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

The chosen policy_max version is the current CMake-version.