In order to LINK with math.lib 
in file LUFA/Build/lufa_build.mk the key "-lm" is added
to the end of the following structure:

.PRECIOUS  : $(OBJECT_FILES)
.SECONDARY : %.elf
%.elf: $(OBJECT_FILES)
	@echo $(MSG_LINK_CMD) Linking object files into \"$@\"
	$(CROSS)-gcc $(BASE_LD_FLAGS) $(LD_FLAGS) $^ -o $@ -lm