Fix cmake warnings on new cmake versions.

CMakeLists.txt

@@ -7,6 +7,9 @@ IF(BIICODE)
 ENDIF(BIICODE)
 cmake_minimum_required(VERSION 2.6 FATAL_ERROR)
 cmake_policy(VERSION 2.6)
+if(POLICY CMP0063)
+	cmake_policy(SET CMP0063 OLD)
+endif()
 
 project(tinyxml2)
 include(GNUInstallDirs)