title: Make warn_unused_result applied to all function with GCC
body: <p>Is it possible to tell GCC to use <strong>warn_unused_result</strong> flag for all function even not having  the corresponding attribute? Because if I can forget to check for return value, I can also forget to add the GCC specific attribute.</p>
  
  <p>I've seen it to be possible with some other compilers.</p>