* CGNS: Add setting of RPATH on Darwin
This should go in the CGNS CMakeLists.txt, but it isn't there yet and won't be in previous versions, so best to handle it here.
If building on darwin/MacOSX, need to set the RPATH so applications can find shared library correctly.
* Restrict MACOSX_RPATH setting
This has now been added to the CGNS distribution, so the setting of `CMAKE_MACOSX_RPATH` should be restricted to current and previous releases and not upcoming releases.
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