Support cmake 4.0

[ByteHamster]

Otherwise I get the following error message:

CMake Error at CMakeLists.txt:23 (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.


-- Configuring incomplete, errors occurred!

[ByteHamster]

@bingmann could you please have a look at this? :) The change should ensure that the project will continue to compile for several years to come, and fix compilation for early adopters who already use cmake 4.0

[ByteHamster]

Thanks!