CMakeLists.txt: Restore policies as we unbumped cmake version

CMakeLists.txt

@@ -1,8 +1,18 @@
 project(OpenMW)
-cmake_minimum_required(VERSION 3.1.0) # CMP0083 NEW
+cmake_minimum_required(VERSION 3.1.0)
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
+# for link time optimization, remove if cmake version is >= 3.9
+if(POLICY CMP0069) # LTO
+  cmake_policy(SET CMP0069 NEW)
+endif()
+
+# for position-independent executable, remove if cmake version is >= 3.14
+if(POLICY CMP0083)
+  cmake_policy(SET CMP0083 NEW)
+endif()
+
 # Detect OS
 include(cmake/OSIdentity.cmake)