# In Debian 9+
apt-get install mingw-w64 libz-mingw-w64-dev


# Clone Qt6 base git repo:
git clone https://code.qt.io/qt/qt5.git qt6-src
cd qt6-src/

# Checkout desired version
git checkout v6.4.3

# Initialize subrepos, we need just qtbase
./init-repository -f --module-subset=qtbase

# Create build directory and configure
mkdir ../qt6-build && cd ../qt6-build && \
cmake -GNinja -DCMAKE_BUILD_TYPE=Release \
	-DQT_FEATURE_vulkan=OFF \
	-DQT_NO_PACKAGE_VERSION_CHECK=TRUE \
	-DCMAKE_TOOLCHAIN_FILE=/path/to/debian-i686-w64-mingw32.cmake \
	../qt6-src/

# Whether you need to disable Vulkan or version check depend on .. things.
# Generally the Qt version on the "host" e.g. Linux should match the
# version you are cross-building, so 6.4.3 on both. In my case I have
# 6.4.2 on Debian host and this works.

cmake --build . --parallel