Added CMAKE_VERSION checking.

cmake version less than 3.1 doesn't have the policy CMP0054.

Author: redboltz

CMakeLists.txt

@@ -1,6 +1,9 @@
 CMAKE_MINIMUM_REQUIRED (VERSION 2.8.6)
 
-CMAKE_POLICY(SET CMP0054 NEW)
+IF ((CMAKE_VERSION VERSION_GREATER 3.1) OR
+    (CMAKE_VERSION VERSION_EQUAL 3.1))
+    CMAKE_POLICY(SET CMP0054 NEW)
+ENDIF ()
 
 PROJECT (msgpack)