GNU(step)Makefile & Windows problem

[Lloyd Dupont]

I tryed to compile a simple project (on Windows) using GNUstep makefile
system with a simple makefile like that:
-------------------------------------
include $(GNUSTEP_MAKEFILES)/common.make

TOOL_NAME  = NMReadWrite
ADDITIONAL_OBJCFLAGS = -Wno-import

NMReadWrite_OBJC_FILES = main.m code.m TreeClasses/NMBranch.m
TreeClasses/NMTool.m

include $(GNUSTEP_MAKEFILES)/tool.make
-------------------------------------

it failed on the compilation of TreeClass/NMBranch.m with an error like
that:
-------------------------------------
Making all for tool NMReadWrite...
 Compiling file main.m ...
 Compiling file code.m ...
 Compiling file TreeClasses/NMBranch.m ...
shared_obj/TreeClasses/NMBranch.d: No such file or directory
make[1]: *** [shared_obj/TreeClasses/NMBranch.o] Error 1
make: *** [NMReadWrite.all.tool.variables] Error 2
-------------------------------------

and indeed, when I looked into shared_obj directory, there is not even a
TreeClasses directory into it.

Is it a bug or a feature of the GNUstep makefile system to not being able to
work with multiple directories for the source?
If it's a bug, is there a workaround?

thanks!