Use relative rpath so that the installation and build dirs are relocatable.

This works by asking cmake to use the "install rpath", but setting that rpath
to be relative.

Thanks a lot to Brad King for the help with CMake!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201921 91177308-0d34-0410-b5e6-96231b3b80d8