"introduce no new bootstrap warning" criteria. was: Loop iv debugging patch

[dewar@gnat.com]

<<I don't think we can demand absolutly no new warnings.  But we could
classify the warnings and demand that some warnings (like the one I
reported and fixed) are forbidden but others are ok
(e.g. signed/unsigned compares).
>>

I prefer a zero warnings criterion for the bootstrap, but for that you
definitely need the feature to selectively turn off warnings for a 
particular section of code.

In GNAT, we have a compiler option to make all warnings fatal, and we
always use this option for all builds. We have also adopted a zero
warnings tolerance for all our C code.

Obviously this cannot happen over night, but I think it is a very
useful goal. There is something about zero warnings that is considerably
more useful than "few" warnings.