Combined tree builds for mingw32

[Danny Smith]

> > 
> > > 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?
> 
> I am just guessing, but I suspect that something before the middle of
> stl_algobase.h caused <windows.h> to be included.  Here's the ugly
> patch I'm using right now to get past this:
> 

You've probably discovered this already, but...

<windows.h> should not be included by default in libstdc++ on mingw
target.
That is taken care of by config/os/mingw32/os_defines.h.  
min/max macros also are taken care of there, but I see the comment is
out-of-date.

Most people I know who build mingw-targeted  gcc/binutils natively or
cross, preinstall the
mingw runtime headers and libraries in $(prefix).  Unlike other systems,
mingw32 does not have 
a standard location for headers or libraries, but uses the
make_relative_prefix dance to find the local dirs.

Danny