[Development] Switch of MinGW toolchain for Qt 5.1

[Axel Waggershauser]

In light of the recent mentioning of gcc's switch to DWARF4 from
DWARF2 with version 4.8, which should help get down the size of debug
builds, I am very much for giving the 4.8.0 option a try.

Cheers,
  Axel