Switch to mingw-build toolchains / Linux distro packages for Windows toolchains

Submitted by Sebastian Dröge (slomo)

Link to original bug (#775777)

Description

https://wiki.qt.io/MinGW-64-bit recommends these here:
https://sourceforge.net/projects/mingwbuilds/files/host-windows/releases/4.8.1/64-bit/threads-posix/seh/
https://sourceforge.net/projects/mingwbuilds/files/host-windows/releases/4.8.1/32-bit/threads-posix/

We should switch to those instead of relying on our very old toolchains that nobody maintains. Or alternatively to these https://sourceforge.net/projects/mingw-w64/files/Multilib%20Toolchains%28Targetting%20Win32%20and%20Win64%29/ray_linn/gcc-6.X-with-ada/ which are also linked from the Qt wiki.

For Linux cross compilation, we should just rely on the distro packages. Both Fedora and Debian have recent packages.

All this just needs some changes all over the place for setting up things correctly inside cerbero. And then we still need msys (or something similar, msys2?) to get wget, flex and bison and to actually get a working shell with all the tools.