cmake: remove duplicate rpath settings for Apple

On OSX 10.9, with Apple Clang 5.1 and cmake 3.0.0 I got an error from install_name_tool complaining about duplicate paths resulting in broken dylibs and executables. The attached patch fixes this and has been verified to not break stuff on OSX 10.8 with Apple Clang 5.1 and cmake 2.8.12 and Ubuntu 12.04, gcc cmake 2.8.10.

Signed-off-by: Ruben Smits <ruben.smits@intermodalics.eu>