Rpath for not APPLE

Created by: andresailer

1. This setting of CMAKE_INSTALL_RPATH got lost in #32
2. I am somewhat sceptical of CMAKE_INSTALL_LIBDIR, because that comes from GnuInstallDirs (right?), bu we don't set this everywhere? But it is used in the APPLE branch?
3. I need something like this for my nightlies put on clicdp cvmfs.

BEGINRELEASENOTES

• RPATH: set rpath to $ORIGIN/../${CMAKE_INSTALL_LIBDIR}

ENDRELEASENOTES