[patch, build] Restore bootstrap in building libcc1 on darwin

[FX]

10-days ping
This restores bootstrap on a secondary target, target maintainer is OK with it. I think I need build maintainers approval, so please review.


> when the freshly built g++ is used, we need to pass the appropriate -B options. As I understand it, the appropriate place for that is in the toplevel configure.ac, where we already pass down the respective -L options. Indeed, the attached patch restores bootstrap on x86_64-apple-darwin14 with gcc as system compiler (and doesn’t break the bootstrap with clang as system compiler).
> 
> OK to commit?
> 
> FX
> 
> 
> 
> PS: HJ, the reason only don’t see this on Linux is that only Darwin (AFAIK) plays spec tricks with static-libstdc++ (in gcc/config/darwin.h)

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