g++ ... -L${exec_prefix}/lib ...

Maybe the ${exec_prefix} hasn't been setup correctly? (Or maybe it didn't get filled in, for whatever reason — it's slightly unusual to have variables in those compile command lines...(because it makes 'post-mortem' debugging of the build process harder))   For testing purposes I would replace it with the actual qt lib directory (or add another -L... directive), and see if you can get the test program built when you manually issue the command...