Support in-tree compilation

Makefile

@@ -20,7 +20,11 @@ ifeq ($(GCC_VER_49),1)
 EXTRA_CFLAGS += -Wno-date-time	# Fix compile error && warning on gcc 4.9 and later
 endif
 
-export TopDIR ?= $(shell pwd)
+ifneq (,$(findstring /usr/lib/dkms,$(PATH)))
+    export TopDIR ?= $(shell pwd)
+else
+    export TopDIR ?= $(srctree)/$(src)
+endif
 
 EXTRA_CFLAGS += -I$(TopDIR)/include