The main CMakeLists.txt had cmake_minimum_required (VERSION 2.8).
This made the cmake to show a deprecation notice that support of all versions below 3.5 will be dropped soon.

The maximum required cmake version is 3.11 for the litehtml module.
So let's set it as a minimum for all other modules.

The FATAL_ERROR option is ignored in CMake >= 2.6.