libstdc++ in a combined tree

[DJ Delorie]

> Well, that sounds like an autoconf bug.  If it refuses to work when
> presented with a pile of compiler options, that just sounds bad.

No, I think it's our bug - we do this:

GCC_NO_EXECUTABLES
GLIBCXX_ENABLE_SYMVERS([yes])

You can't logically expect that to work, no matter how many compiler
options we give it - we've told autoconf to not produce any
executables, then asked it to test something that requires them.

> If you're looking for a short-term hack, the best I can think of is
> --enable-libstdc++-symvers.  I think (but I'm not a libstdc++
> maintainer!) that an --enable-libstdc++-symvers option might be
> acceptable, and that wouldn't run afoul of my concern about using
> autoconf only in native configurations, etc.

I suppose a small case statement could make reasonable guesses about
defaults, too.  *-linux* yes, *-elf no, etc.