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

This reverts commit r201934.

Polly has been fixed to work with this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201936 91177308-0d34-0410-b5e6-96231b3b80d8