Qt fails to build due to Project ERROR: Unknown module(s) in QT: quick-private

When cross-compiling Qt 5.12.0 Alpha using x86_64-w64-mingw32 7.3.0 as provided by Ubuntu 18.04 (as well as Debian Testing & Unstable), the build fails with the following error:

Project ERROR: Unknown module(s) in QT: quick-private
Makefile:124: recipe for target 'sub-distancefieldgenerator-make_first' failed
make[2]: *** [sub-distancefieldgenerator-make_first] Error 3
make[2]: *** Waiting for unfinished jobs....

To reproduce, run the attached build_qt.sh script on Ubuntu 18.04 with x86_64 as the argument:

./build_qt.sh x86_64

If you prefer using Docker:

mkdir build
mv build_qt.sh build
sudo docker run --rm -v "$PWD/build":/build ubuntu:18.04 /bin/bash /build/build_qt.sh x86_64

p.s.: the failing project is qttools/src/distancefieldgenerator but there doesn't seem to be a Component section for such random tools on Jira, so I have put it under Core: Other for not knowing any better.