fix cmake 3.5 issue

[domibel]

This patch should help with the transition to CMake 4.0 #275

[domibel]

All checks have passed. Can this be merged please?

[leochan2009]

Hi @domibel, hi @Sunderlandkyl ,

can we have something like this to make the code compilable under different system with older Cmake version?
if(CMAKE_VERSION VERSION_LESS "3.5.0")
message("Please consider to switch to CMake 3.5.0 or newer")
cmake_minimum_required(VERSION 2.8.11)
endif()

[domibel]

CMakeLists.txt is already set to
CMAKE_MINIMUM_REQUIRED(VERSION 3.1)
because of
(CMAKE_CXX_STANDARD 11)

So it should be safe to bump the version slightly up to at least 3.5 in all CMakeLists.txt files.