fix a issue that libclang.dylib won't be loaded when it is linked with libLLVM-3.5.dylib

When I try to use ycmd with homebrewed libclang where `libclang.dylib` is linked to `libLLVM-3.5.dylib`, the `ycm_core.so` won't load (error message: `dyld: no suitable image not found`).

For macs, we can have multiple RPATHs. To my surprise, appending the dependent library's path to the `ycm_core.so`'s rpath list just solves this issue. 

BTW passing the argument `-Wl,rpath,EXTRA_LIB_PATH` to the compiler should produce the same effect as the command `install_name_tool -add_rpath` does.