Permit CMAKE_INSTALL_RPATH to be set on command line

Commit 201921 overrides setting of CMAKE_INSTALL_RPATH via the
command line. Last time this happened we applied another patch
to only set CMAKE_INSTALL_RPATH if already defined (r197825).
This patch does the same thing again, but only for the UNIX
case - we leave APPLE alone as presumably the original committer
is happy with the non-overriding behaviour.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202085 91177308-0d34-0410-b5e6-96231b3b80d8