CMake: Accept <Package>_ROOT from environment

CMakeLists.txt

@@ -2,6 +2,10 @@ cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
 
 message(AUTHOR_WARNING "If the build fails, please try the autotools/qmake method.")
 
+if(POLICY CMP0074)
+    cmake_policy(SET CMP0074 NEW)
+endif()
+
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules)
 include(FunctionReadVersion)