Qmake: Add variable choosers where variables are parsed

src/plugins/autotoolsprojectmanager/makestep.cpp

src/plugins/projectexplorer/buildstep.cpp

src/plugins/projectexplorer/buildstep.h

src/plugins/projectexplorer/processstep.cpp

src/plugins/qmakeprojectmanager/makestep.cpp

src/plugins/qmakeprojectmanager/qmakestep.cpp