Create Mingw-builds based packages with Qt 4.8.6

The binary MinGW packages for 4.8.0-4.8.5 were built with an ancient gcc 4.4 from Mingw.org, which is not available any more (except for dubious sources). We should therefore build Qt 4.8.6 packages with a more recent mingw, ideally the same toolchain we're building the Qt 5.1/5.2 packages with:

http://sourceforge.net/projects/mingwbuilds/files/host-windows/releases/4.8.0/32-bit/threads-posix/dwarf/x32-4.8.0-release-posix-dwarf-rev2.7z/download