#236 build breaks with cmake 4

[Thomas Klausner]

cmake 4 deprecated support for versions < 3.5.
This breaks the build:

-- Detecting CXX compile features - done
CMake Error at CMakeLists.txt:20 (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.

The fix is to increase the version number in CMakeLists.txt:

CMAKE_MINIMUM_REQUIRED(VERSION 2.4.0 FATAL_ERROR)

to at least 3.5.