Added support to RPATH for CMake >= 2.8.12

CMake 2.8.12 changed how `RPATH` is handled (at least for OS X). Furthermore in CMake 3.0 a warning is issued if these changes are not integrated (or the policy disabled).

Using `RPATH` avoids messing with the linker search path in environmental variables.

My patch should be backcompatible: `RPATH` is disabled by default and is supported only on CMake >= 2.8.12.
In case CMake is less recent or (by default) `RPATH` is not chosen, it behaves as before (so it keeps also the CMake warning).

Compiling and installing with `RPATH` on OS X gives the following results:
```bash
$ otool -D liborocos-kdl.dylib 
liborocos-kdl.dylib:
@rpath/liborocos-kdl.1.3.dylib
```
In this way the library is relocable (the library name does not contain the absolute path to the library).

Just one note in this PR:
Lines from 88 to 99 are needed only if KDL links to other non-system libraries (which currently it does not). But they should not harm if kept.