A separate cross compiler mkspecs file (win32-x-g++) is now used to
configure all build options so the setup-mingw-cross-compiling hack is
not needed anymore. Furthermore, the MinGW setup is now on path, so no
need to specify extra PATH changes for it.
It looks like Qt does not support SVG format by default on Windows and
it was not trivial to add the plugin into the build, so for now, build a
16x16 PNG icon file for Windows binary to avoid showing an invisible
icon in the tray.
