Here are the full instructions:

Get it:

git clone https://github.com/mxe/mxe.git

Install build dependencies

Build Qt 5 for Windows:

cd mxe && make qtbase

This will first build its dependencies and the cross-build tools; It should take less than an hour on a fast machine with decent internet access.

Due to the new modular nature of Qt 5, various major Qt components are now in different tarballs. The one selected above, qtbase, should give you enough functionality to run ordinary GUI apps, which is all I needed for my own (smallish) app.

If you want to build all of Qt 5 instead, you'll need to run make qt5 (instead of make qtbase). Note that it will take a lot longer to complete, so be sure that you need the extra functionality.

Get to the directory of your app, and run the Qt Makefile generator tool:

<mxe root>/usr/bin/i686-w64-mingw32.static-qmake-qt5

Build your project:

make

You should find the binary in the ./release directory:

wine release/foo.exe

Some notes:

This was tested on my 64-bit Debian 8, and on Windows of course.

The output is a 32-bit static executable, which will work well on 64-bit Windows.

If you want a 64-bit executable, build Qt with:

make MXE_TARGETS=x86_64-w64-mingw32.static qtbase

The default MXE_TARGETS value is i686-w64-mingw32.static.