Set cmake_policy() to silence warning by CMake 3.x

Fixes #56

CMakeLists.txt

@@ -1,4 +1,9 @@
 cmake_minimum_required(VERSION 2.4)
+if(COMMAND cmake_policy)
+    if(POLICY CMP0026)
+        cmake_policy(CMP0026 OLD)
+    endif()
+endif()
 project(postsrsd C)
 include(CheckIncludeFile)