[Bf-committers] For MinGW builders

[Antony Riakiotakis]

Hi, just finished moving the gcc libs to a separate lib folder.  I
don't think that the main windows folder got a lot smaller (about 200
MB smaller I think) but the mingw32 folder is just 600MB, so there's a
win there.

MinGW builders will need to check out the lib/mingw32 folder from now
on. The build doc page is also updated to reflect the changes. I hope
people are happy with the result.