On my macintosh, when using the precompiled version of Qt422, qmake
automatically generates... a xcode project.

I know that by using 
        qmake project.pro -spec macx-xcode
you ask qmake to generate an xcode project file...

But... how may I force qmake to generate a simple, old, good Makefile
instead of an xcode project file ?

The secret is in the qmake options:
			qmake -spec macx-g++

qmake -project
qmake -spec macx-g++
make