Explicitly use (new) default OS X rpath policy if present.

- CMake 3.0+ prefers the use of @rpath, and CMake 3.2+ produces a
  developer warning if this policy is not set, thus if present we
  explicitly specify the new default behaviour.
- http://www.cmake.org/cmake/help/v3.2/policy/CMP0042.html
- Also remove unnecessary check for presence of cmake_policy(), as it
  exists in all versions of CMake >= 2.8 (our minimum required version).

Change-Id: Iacbf8186bee4bf07d8a53068fd528cd6237c5efb