Warning for different pointer signedness

[Giovanni Bajo]

Mike,

since your patch: http://gcc.gnu.org/ml/gcc-patches/2004-07/msg01284.html has a
great impact on existing codebases (linux kernel guys say it triggers 6.5K
times in full build), would you mind adding an entry about this into
changes.html? It would help also to put a rationale there on why such a change
was made, since we will undoubtly get this asked many times.

Thanks
Giovanni Bajo