top-level configure: preserve quotes in arguments (Was: Re: [libstdc++] libstdc++/7691, pass -Wabi, sanity checks)

[Benjamin Kosnik]

> Hmm...  There's one thing I dislike about your patch, which is that it
> turns --enable-cxx-flags= into --enable-cxx-flags (with an implicit
> =yes).  I'd rather set addoption to the entire string to be added to
> $arguments, quoted if necessary.

I agree. Thoughts? Phil, what do you think? Does this look ok?

-benjamin