Permit CMAKE_INSTALL_RPATH to be set on command line
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@202085 91177308-0d34-0410-b5e6-96231b3b80d8

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