CMakeLists: backwards compatibility with CMake < 3.0.

contrib/zlib/CMakeLists.txt

@@ -1,9 +1,12 @@
 cmake_minimum_required(VERSION 2.4.4)
 set(CMAKE_ALLOW_LOOSE_LOOP_CONSTRUCTS ON)
 
-# use the old project command http://www.cmake.org/cmake/help/v3.0/policy/CMP0048.html
+# CMake 3.0 changed the project command, setting policy CMP0048 reverts to the old behaviour.
+# See http://www.cmake.org/cmake/help/v3.0/policy/CMP0048.html
 cmake_policy(PUSH)
-cmake_policy(SET CMP0048 OLD)
+if(CMAKE_MAJOR_VERSION GREATER 2)
+	cmake_policy(SET CMP0048 OLD)
+endif()
 project(zlib C)
 cmake_policy(POP)