Compatibility with CMake < 3.5 will be removed from a future version of CMake

[kosude]

When configuring with cmake . -Bbuild/, the following warning presents:

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.

This occurs for the CMakeLists.txt file in the root project dir as well as in zf_log/. Updating the minimum version to at least 3.5 might be worth considering in order to keep support with newer CMake versions in the future.