-Wextra-strict-aliasing proposal

[Jim Wilson <wilson at specifixinc dot com>]

This is a continuation of my earlier message for a -Wstrict-aliasing
extension.
    http://gcc.gnu.org/ml/gcc-patches/2004-02/msg02578.html

Since the earlier patch, I have changed "may" to "might".

I made the new warning depend on a new option -Wextra-strict-aliasing. 
The warning can not be part of -Wstrict-aliasing, because that option is
enabled by -Wall.  Since the new warning is sometimes emitted for code
that is actually safe, it seems inappropriate for this to be part of
-Wall.  In particular, there are some safe casts in the gcc source code
that trigger this warning, and it would be annoying to get them during
gcc bootstraps which use -Wall.  For the new option name, I was thinking
-Wextra as opposed to -Wall, where -Wextra is documented to emit extra
warnings you might not want, plus it makes some sense as an English
phrase.

I added documentation and a testcase since the earlier patch.

This was tested with an x86-linux bootstrap and make check, there were
no regressions.

I plan to add this in a few days unless someone has an objection to it.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com

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