RFC: should we use -Werror? (& sample patch to do it)

[dewar@gnat.com]

<<Well, the control is not that fine grained, even for warnings, since is
is not possible to disable a warning locally, but only for a full
translation unit.
>>

In GNAT, we have a pragma Warnings (On|Off) which can be applied locally
and is very useful for eliminating "OK" warnings (in GNAT we require
compilation with -gnatwe which is the equivalent of -Werror.