Avoid multiple multi-do/multi-clean rules

[Loren James Rittle]

> 2003-03-14  Andreas Schwab  <schwab@suse.de>
>
>	* configure.in: Only append to makefiles that are newly created to
>	avoid multiple multi-do/multi-clean rules.
>	* configure: Rebuilt.

Thanks you!  We, libstdc++-v3 maintainers, had wondered how to fix
that long-standing problem.  If you didn't commit as obvious (in
retrospect), approved for mainline!  (Although very annoying to
developers, the issue being solved doesn't cause any real failures
thus I'd lean against putting it anywhere else.) - Loren