Avoid CMP0048 warning (#570)

Fixes #567.

If pybind's CMakeLists gets loaded via an include_directory from another
CMakeLists with a higher minimum version (e.g. 3.0), the project()
command without a version produces a CMP0048 warning.

This commit explicitly requests the new behaviour if the policy exists,
as it won't cause problems (we set VERSION later).

CMakeLists.txt

@@ -7,6 +7,11 @@
 
 cmake_minimum_required(VERSION 2.8.12)
 
+if (POLICY CMP0048)
+  # cmake warns if loaded from a min-3.0-required parent dir, so silence the warning:
+  cmake_policy(SET CMP0048 NEW)
+endif()
+
 project(pybind11)
 
 # Check if pybind11 is being used directly or via add_subdirectory