sdl2-compat: cmake: use if(POLICY) to check for policy existence

SDL Commits
1 
From 67fe2020058beea463eff34af3c58f01b3dac036 Mon Sep 17 00:00:00 2001
From: Anonymous Maarten <[EMAIL REDACTED]>
Date: Tue, 29 Nov 2022 23:47:22 +0100
Subject: [PATCH] cmake: use if(POLICY) to check for policy existence

---
 CMakeLists.txt | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 21bb0c0..dba5978 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -128,8 +128,7 @@ if(APPLE)
 #  set(DYLIB_CURRENT_VERSION 12.61.0 CACHE STRING "library current version")
   find_package(X11)
   include_directories("${X11_INCLUDE_DIR}")
-  if(CMAKE_VERSION VERSION_LESS 3.9)
-  else()
+  if(POLICY CMP0068)
     cmake_policy(SET CMP0068 NEW)  # on macOS, don't let RPATH affect install_name.
   endif()
 endif()