parameterize kernel directory

The directory for kernel headers can now be
overridden through the KERNEL_DIR variable

Makefile

@@ -1,4 +1,5 @@
 KERNELRELEASE ?= $(shell uname -r)
+KERNEL_DIR ?= /lib/modules/$(KERNELRELEASE)/build
 
 DEBUG ?= n
 ifeq ($(DEBUG),y)
@@ -7,7 +8,7 @@ else
 	EXTRA_CFLAGS += -O2
 endif
 
-EXTRA_CFLAGS += -I$(PWD)/include
+EXTRA_CFLAGS += "-I$(PWD)/include"
 
 obj-m += keymash.o
 
@@ -16,10 +17,10 @@ keymash-sources = src/keymash.c
 keymash-objs = $(keymash-sources:.c=.o)
 
 keymash.ko:
-	make -C "/lib/modules/$(KERNELRELEASE)/build" M="$(PWD)" modules
+	make -C "$(KERNEL_DIR)" M="$(PWD)" modules
 
 clean:
-	make -C "/lib/modules/$(KERNELRELEASE)/build" M="$(PWD)" clean
+	make -C "$(KERNEL_DIR)" M="$(PWD)" clean
 
 all: keymash.ko