make: ****** empty variable name. Stop.

This is make file for rtdm

obj-m           := tut01-skeleton-drv.o
KDIR            := /lib/modules/$(shell uname -r)/build
PWD             := $(shell if [ "$$PWD" != "" ]; then echo $$PWD; else pwd; fi)
EXTRA_CFLAGS := -I$(KDIR)/include/xenomai -I$(KDIR)/include/xenomai/posix           $(ADD_CFLAGS)

all:     
        $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
clean:
        $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) clean

Note

-I had kept space between = and $ that caused this error. Remove the space and error would be solved

Make file
$=means value
#=number of