CMakeLists.txt: Raise minimum version to 4.0

In July 2023, CMake deprecated backwards compatibility for versions older
than 3.5. CMake 4.0 finally drops support, causing all packages still
relying on older behaviour to fail to build from source.

No changes are needed with the higher version requirement.

Author: Hugh McMaster

CMakeLists.txt

@@ -3,7 +3,7 @@
 # Modifications Copyright (C) 2019 Matthias C. Hormann <mhormann@gmx.de>
 # This file is released under the 2 clause BSD license, see COPYING
 
-CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
+CMAKE_MINIMUM_REQUIRED(VERSION 3.31...4.0)
 PROJECT(loudgain C CXX)
 
 SET(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/" ${CMAKE_MODULE_PATH})