Set CMake policies for Apple Clang/RPATH

CMake 3 introduces following features:

- Compiler id of Clang on Mac is now "AppleClang" to distinguish it
- All shared lib targets on OS X use the @rpath loader path (see
  http://www.kitware.com/blog/home/post/510). Support added
  in 2.8.12

Running CMake 3 will emit warnings about these policies.

As FNALCore requires CMake 2.8.12, set:

- Clang policy so that OLD functionality is used, that is Clang is
  Clang.
- RPATH policy set so that Mac always uses @rpath path