qmake: When using an env var inside OBJECTS_DIR it will not be correctly placed in the Makefile if the env var is not set

qmake: When using an env var inside OBJECTS_DIR it will not be correctly placed in the Makefile if the env var is not set at qmake time

Using OBJECTS_DIR = .obj$(DEBUGSUF) in .pro file
and calling
make DEBUGSUF=_debug

produces

.cpp.o:
$(CXX) -c -include .obj/pchtest$(DEBUGSUF) $(CXXFLAGS) $(INCPATH) -o "$@" "$<"

instead of
.cpp.o:
$(CXX) -c -include .obj$(DEBUGSUF)/pchtest$(DEBUGSUF) $(CXXFLAGS) $(INCPATH) -o "$@" "$<"
in the Makefile.