Combined tree builds for mingw32

[Paolo Carlini]

Hi,

>The #undef min / #undef max in c++config.h do not work if c++config.h
>is included before windows.h, which happens during a build of the PCH;
>  
>
I'm not sure to understand. Can you explain in better detail? 
Proviously, we had those undefs in the middle of stl_algobase.h, I don't 
see why that was better. Also, I don't understand what the PCHs have to 
do with windows.h

>with that hacked around, I still get:
>
>/space/fsf/mingw/obj/i586-mingw32msvc/libstdc++-v3/include/i586-mingw32msvc/bits/ctype_base.h:59:
>error: '_N' was not declared in this scope
>
>Nothing pulls in the definition of _U, _N, etc before that point.
>  
>
Before *which* point? AFAICS, the only place where ctype_base.h is 
included is in bits/locale_facets.h, *after* <cctype>...

Paolo.