Avoid warning in CMake when using re2 as a subproject

Change-Id: Iebd37f2367a29db386a20a7686b93c54c3b2c7cd
Reviewed-on: https://code-review.googlesource.com/18071
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3e22472..884873b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,6 +5,10 @@
 # Old enough to support Ubuntu Trusty.
 cmake_minimum_required(VERSION 2.8.12)
 
+if(POLICY CMP0048)
+  cmake_policy(SET CMP0048 NEW)
+endif()
+
 project(RE2 CXX)
 include(CTest)