Origin is comment at https://codereview.qt-project.org/c/qt/qtbase/+/298509/10/qmake/CMakeLists.txt#255
If you look at the following failed qtremoteobjects integration
http://testresults.qt.io/coin/integration/qt/qtremoteobjects/tasks/web_qt_qtremoteobjects_1592323040002 which was done for PS2 of https://codereview.qt-project.org/c/qt/qtremoteobjects/+/304624/2
you can see that CMake fails to configure when building for iOS with the following messages:
qt_process_qlalr is one of the functions that didn't check if the target is an imported target and should do nothing, similar to qt_extend_target.
While that is addressed in https://codereview.qt-project.org/c/qt/qtbase/+/304656
the main issue remains that we shouldn't have to do this, and we can't do this for non qt_ prefixed functions.
We should instead introduce a function like qt_tool_will_be_built() and teach pro2cmake to use it and exit the add_subdirectory() call early in case it isn't being built.