Combined tree builds for mingw32

[Paolo Carlini]

Paolo Carlini wrote:

>> The failing command is trying to compile the PCH.  This means that
>> we're including a large number of libstdc++ headers in a row.  One of
>> the first ones pulls in c++config.h, which has #undef max; but so far,
>> nothing has included <windows.h>.  Later, something includes
>> gthr-default.h; on this platform that pulls in <windows.h>, which
>> eventually #define's min.  Then later we get <limits>, which now sees
>> the definition. 
>
> I see, but I still don't get why this is worse than time ago, when the 
> undefs where placed in the middle of stl_algobase.h and nowhere else. 
> Do you?

Forgot: in fact, I don't see why the specific scenario above can happen: 
<limits> now includes c++config.h, which (now, not in old releases) 
undefs #min and #max again...

Paolo.