Updated RPATH behaviour

I changed how RPATH is handled both in OS X and in Linux.

- For CMake < 2.8.12 everything should be the same as before (even if the changes on the linux size should work on older versions of cmake).
- For CMake >= 2.8.12 the changes are the following:
-- OS X: enabled real support to RPATH introduced in CMake 2.8.12
-- Linux and OS X: rpath is now relative to the executable. In this way moving the install folder (the one containing `lib` and `bin`) should keep intact the link paths.

Of course RPATH is still off by default.

I tested it on OS X, CMake version 2.8.12 and 3.0.1. I need some more test configurations (e.g. Linux with CMake 2.8.11 and a newer version).

I just remembered that `${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}` could be replaced by `${CMAKE_INSTALL_FULL_LIBDIR}` (here and in the `file(RELATIVE_PATH _rel_path [...]` command that I suggested).