clangbackend: Add missing rpath setting in qbs product.

And another linker flag that was present in the qmake project file, but
not in the qbs one.

Change-Id: I703d5df2171cf97974111993aec8ed244435afec
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/tools/clangbackend/clangbackend.qbs

@@ -29,4 +29,10 @@ QtcTool {
     cpp.includePaths: base.concat(["ipcsource", llvmIncludeDir])
     cpp.libraryPaths: base.concat(llvmLibDir)
     cpp.dynamicLibraries: base.concat(llvmLibs)
+    cpp.rpaths: base.concat(llvmLibDir)
+
+    Properties {
+        condition: qbs.targetOS.contains("unix") && !qbs.targetOS.contains("osx")
+        cpp.linkerFlags: base.concat(["-z", "origin"])
+    }
 }