Build on Windows with MSYS2

Install MSYS2 from https://www.msys2.org/ .

Next, run:

pacman -Syu
pacman -S --needed base-devel mingw-w64-x86_64-toolchain
pacman -S --needed mingw-w64-x86_64-qt5

Run "MSYS MinGW 64-bit" from Start menu:

...

WIP..