Fix build failures with CMake 4
https://bugs.gentoo.org/952092
--- a/CMakeLists.txt	2025-03-26 21:03:28.124742115 +0300
+++ b/CMakeLists.txt	2025-03-26 21:03:56.696915295 +0300
@@ -40,7 +40,7 @@
 #                        By default ArpON will be installed to / path.
 #
 
-cmake_minimum_required(VERSION 2.6)
+cmake_minimum_required(VERSION 3.5...4.0)
 
 project(ArpON C)
 set(AUTHOR "Andrea Di Pasquale <spikey.it@gmail.com>" INTERNAL "Author")