CMake: Re-enable LTO for Release builds with CMake >= 3.9

If you're using an earlier version, I suggest you upgrade, CMake 3.9 was
released in 2017....

CMakeLists.txt

@@ -145,7 +145,19 @@ if(MSVC)
 
   # Enable LTO/LTCG on Release builds.
   if(${CMAKE_BUILD_TYPE} STREQUAL "Release")
-    set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON)
+    if (${CMAKE_VERSION} VERSION_LESS "3.9.0")
+      message(WARNING "CMake version is less than 3.9.0, we can't enable LTCG/IPO. This will make the build slightly slower, consider updating your CMake version.")
+    else()
+      cmake_policy(SET CMP0069 NEW)
+      include(CheckIPOSupported)
+      check_ipo_supported(RESULT IPO_IS_SUPPORTED)
+      if(IPO_IS_SUPPORTED)
+        message(STATUS "Enabling LTCG/IPO.")
+        set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON)
+      else()
+        message(WARNING "LTCG/IPO is not supported, this will make the build slightly slower.")
+      endif()
+    endif()
   endif()
 endif()