this is was I was looking for and gives me the correct results that I am expecting.

Sorry, I have readed too fast your post! Yes, using requires() is the right way to do what you want!

Well, my approach was even wrong in the first place.
What I wanted to achieve is done via one line in the *.pro file:

requires(qtHaveModule(quick))

this is was I was looking for and gives me the correct results that I am expecting.

I simply want to get at least a kind of understanding of what goes on here.

As describe here ==> https://doc.qt.io/qt-5/qtgui-index.html
GUI module is included by default and must be removed if not required/used

However, I do not understand why the error appears in the first place. Because qmake should not process the file at all.
I have several other project files, some of them contain QT-=gui and some of them don't, however they all build successfully, even those where I did not put that in.

Furthermore, the project file in question is practically empty beside the qthavemodule check, so why tries qmake to do something else afterwards.

I simply want to get at least a kind of understanding of what goes on here.

Project ERROR: Unknown module(s) in QT: gui

If you don't you gui, you should insert in your PRO file QT -= gui

qtHaveModule(quick){
DEFINES += \
	BLABLA_UI_LIBRARY
QT += quick

[...]
}
else
{
    message("QtQuick not available, skipping building the blabla-ui lib")
    QT -= gui
}