Bump minimum version to fix build with CMake4
https://bugs.gentoo.org/951931
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -20,7 +20,7 @@
 #
 #top level cmake project for libbtbb lib + tools
 
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.5)
 project(libbtbb_all)
 set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/modules)