Use an install name dir of @executable_path/../lib instead of @rpath.

Using @executable_path/../lib matches what we have on Makefiles and works
with older versions of OS X too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202302 91177308-0d34-0410-b5e6-96231b3b80d8