cmake: test for CMP0051 instead of version check

We can use policy checks to see if a policy exists in cmake, like
CMP0051, instead of relying on the version.

Author: ethomson

CMakeLists.txt

@@ -14,7 +14,7 @@
 PROJECT(libgit2 C)
 CMAKE_MINIMUM_REQUIRED(VERSION 2.8.11)
 CMAKE_POLICY(SET CMP0015 NEW)
-IF (NOT CMAKE_VERSION VERSION_LESS 3.1)
+IF (POLICY CMP0051)
 	CMAKE_POLICY(SET CMP0051 NEW)
 ENDIF()