use a relative rpath for rpath (to keep it relocatable)

CMakeLists.txt

@@ -65,9 +65,11 @@ cmake_path(APPEND module_dir "${qtqmldir}" "${module_target_path}")
 message(STATUS "Module dir ${module_dir}")
 
 # allow the installed plugin to find its backing library
+cmake_path(SET plugin2backinglibpath ${qtlibdir})
+cmake_path(RELATIVE_PATH plugin2backinglibpath  BASE_DIRECTORY ${module_dir})
 set_property(
     TARGET "${module_plugin_target}"
-    APPEND PROPERTY INSTALL_RPATH "${qtlibdir}"
+    APPEND PROPERTY INSTALL_RPATH "\${ORIGIN}/${plugin2backinglibpath}"
 )
 
 # Install the QML module runtime loadable plugin