bootstrap/7090: libstdc++ configure script fails to find install-sh when cross-compiling

[DJ Delorie]

> That looks like a different PR I already fixed.  If you can find the
> duplicate, you can link them.  They might need to try 3.1.1 instead of
> 3.1, too.

Of course, now that I've *read* the whole thing, I realize that THAT
is the PR I worked on.  Nevermind.  Yeah, Scott needs to confirm that
it's fixed, and then close it.  IMHO only the originator should close
PRs they opened.