To reproduce:

git clone --depth 1 -b 5.8 git://code.qt.io/qt/qtbase.git
git clone --depth 1 -b 5.8 git://code.qt.io/qt/qtxmlpatterns.git

cd qtbase && ./configure -release -no-libinput -no-linuxfb -no-xcb \
-no-kms -no-gbm -no-gtk -no-widgets -no-compile-examples \
-opensource -confirm-license -make libs \
-prefix /opt/qt

make -C qtbase
sudo make -C qtbase install
cd qtxmlpatterns && /opt/qt/bin/qmake

Outputs the message:

Qt is now configured for building. Just run 'make'.
Once everything is built, Qt is installed.
You should NOT run 'make install'.
Note that this build cannot be deployed to other machines or devices.

Afaict this is incorrect. "make install" should be run to install the built module into /opt/qt.

This is caused by https://code.qt.io/cgit/qt/qtbase.git/commit/?id=cfba3fff052b0af411d21727a68e78ea3e6a21d0

That commit says "another effect of this is that repositories outside qtbase will now also get the installation note in modular builds, which makes sense."

Which is fine, except that the wrong installation note is printed.

Digging deeper the logic says:

pfx = $$[QT_INSTALL_PREFIX]
equals(pfx, $$[QT_INSTALL_PREFIX/get]) {

ie if QT_INSTALL_PREFIX == QT_INSTALL_PREFIX/get, then tell the user not to "make install". However, in every version of qmake that I checked, these two vars are equal.