#14 build breaks with cmake 4

[Thomas Klausner]

cmake 4 deprecated support for versions < 3.5, see https://cmake.org/cmake/help/v4.0/release/4.0.html#deprecated-and-removed-features
This breaks the build of ebook-tools:

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.

The fix is to increase the version number in CMakeLists.txt to at least 3.5:

cmake_minimum_required(VERSION 3.5)