Ok, here is the point:
the diff between the generated makefile ( through QtCreator ) and the one generated by my command line is:
INCPATH = -I/Soft/fox_dev/Qt-4.7.4/mkspecs/linux-g++-32 -I. -I/Soft/fox_dev/Qt-4.7.4/include/QtCore -I/Soft/fox_dev/Qt-4.7.4/include/QtXml -I/Soft/fox_dev/Qt-4.7.4/include/QtXmlPatterns -I/Soft/fox_dev/Qt-4.7.4/include -I../../../fx/libfx -I../../../fx/MQLib -I/Soft/fox_dev/boost/boost_1_54_0 -I/Soft/fox_dev/xerces-c-3.1.1-x86-linux-gcc-3.4/include -I/Soft/fox_dev/xerces-c-3.1.1-x86-linux-gcc-3.4/include -I.
Vs
INCPATH = -I/Soft/fox_dev/Qt-4.7.4/mkspecs/linux-g++ -I../fxa_march -I/Soft/fox_dev/Qt-4.7.4/include/QtCore -I/Soft/fox_dev/Qt-4.7.4/include/QtXml -I/Soft/fox_dev/Qt-4.7.4/include/QtXmlPatterns -I/Soft/fox_dev/Qt-4.7.4/include -I../../../fx/libfx -I../../../fx/MQLib -I/Soft/fox_dev/boost/boost_1_54_0 -I/Soft/fox_dev/xerces-c-3.1.1-x86-linux-gcc-3.4/include -I/Soft/fox_dev/xerces-c-3.1.1-x86-linux-gcc-3.4/include -I. -I../fxa_march -I.
however one can go to "Project-> Build Settings -> [choose configuration to modify] -> And edit the "Qmake step" with additionnal argument:
-spec linux-g++-32
you will also need to add in the .pro file either of:
CFLAGS += -m32
QMAKE_CXXFLAGS += -m32