CMakeLists.txt: update CMake minimum required version

Building with CMake >= 4.x requires the cmake_minimum_required() version to be at least 3.5, so let's update to that to fix:

CMake Error at CMakeLists.txt:3 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

Signed-off-by: Bernd Kuhls bernd.kuhls@t-online.de