Avoid multiple multi-do/multi-clean rules

[Benjamin Kosnik]

>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

Agreed! This has long annoyed me, and all my previous attempts broke
multi-libbed archs like AIX, etc.

Thanks a bunch Andreas.

(now if we can just get this annoying -g -O2 -g -O2 -g -O2 stuff
licked... when did this weirdness creep in? What is going on with the
configure/make patches?!!!!)

-benjamin