Greetings emacs-devel,

Attached is a patch that adds the variable `byte-compile-error-on-warn'
to the safe variable list temporarily during calls to
`byte-compile-file`, so that individual files can easily and simply
include "byte-compile-error-on-warn: t" in their file local variables.

This allows programmers to be able to ensure thoroughly that there are
no warnings in their code, which theoretically leads to better
correctness.

It is my eventual hope that this is accepted so that
`byte-compile-error-on-warn' can be enable on files in the core Emacs
Git repository, extending the aforementioned correctness potential to
files in the core.

For example, something that is bothering me right now
is that there are quite a few 'foo is obsolete' warnings during the byte
compilation portion of the build phase. Enabling error on warning means
that trying to byte compile files that reference stale
functions/variables/what have you will fail, prompting patches that
obsolete Lisp objects to also fix usage of it in the core with the
appropriate recommended replacement.

I ran the full test suite (`make check-expensive`), and got the same
results pre- and post-patch. Unless there are already files that have
`byte-compile-error-on-warn' set in their local variables (which
wouldn't have worked prior to this), nothing should have suddenly
broken as a result of this patch.

Suggestions, comments, concerns, and all those other things are welcomed
and requested.

Thanks,
-- 
~Robert Cochran

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