setup Rpath for MacOS

cmake/os.cmake

-## OSX/homebrew version of root6 installs its cmake macros in a non-standard
-## location. This might be an issue on other systems as well.
-if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
-  set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} $ENV{ROOTSYS}/etc/root/cmake)
-endif()
-
-## Get rid of rpath warning on OSX
+## Set correct rpath on MacOs
 if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
   set(CMAKE_MACOSX_RPATH 1)
+  set(CMAKE_INSTALL_RPATH
+    ${CMAKE_INSTALL_RPATH} 
+    "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
+message("Setting RPath to ${CMAKE_INSTALL_RPATH}")
 endif()