Upgrade "may not respond to" warning to error

[Scott Lamb <slamb at slamb dot org>]

I'd like to have warnings like this:

    someFile.m:XXX:warning: `SomeObjcClass' may not respond to 
`-someMessage:'

become errors, while not affecting other warnings. So I have two 
questions:

1) What's the flag for this message? I see -Wundeclared-selector, but 
that seems to be a slightly different case.

2) Is it possible to do something like -Werror on just some warnings? 
I'd like some other types of warnings (say, the signedness ones) to be 
just warnings. I want to see them, but I don't want them to cause 
compilation to fail.

Thanks,
Scott

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