[patch libstdc++]: Fix PR/57212

[Benjamin De Kosnik]

> 2013-05-08  Andoni Morales Alastruey <ylatuya@gmail.com>
> 
>     PR libstdc++/57212
>     * include/Makefile.am (_GLIBCXX___MINGW32_GLIBCXX___): Modify
>     to __MINGW32__.
>     * include/Makefile: Regenerated.
> 
> I will apply this patch as obvious fix for Andoni.

Thanks Kai!

-benjamin