Support cmake 4.0 without workarounds

[brawer]

When building libosmium 2.22.0 with cmake 4.0, configuration fails like this:

CMake Error at CMakeLists.txt:9 (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!
>>> ERROR: libosmium: build failed

See command line invocation and error log. As a workaround, users can pass -DCMAKE_POLICY_VERSION_MINIMUM=3.5, which we'll do for now when packaging libosmium for Alpine Linux. However, consider updating CMakeLists.txt, so people can build libosmium with modern CMake without trouble.