test if compiler warns

[Peter Johansson]

Hi autoconfers,

In one of my projects, we noticed that some versions of g++ does not 
recognize option '-pthread', but gives a warning

i686-apple-darwin8-g++-4.0.1: unrecognized option '-pthread'

Now I'd like to test this at configure time, so I wonder if there is a good 
mechanism to test if compiler is complaining to stderr.
Or do I need to write a low-level macro myself for this?

Thank you.

Peter