avoid make jobserver warnings

Makefile

@@ -19,16 +19,16 @@ MAKEFLAGS += --no-print-directory
 .PHONY:	all clean initrd apps grub_image install install_img depend copy_image bochs bochsq
 
 all:
-	make -C kernel
-	make -C rmmod
+	${MAKE} -C kernel
+	${MAKE} -C rmmod
 
 clean:
-	-make -C kernel clean
-	-make -C rmmod clean
+	-${MAKE} -C kernel clean
+	-${MAKE} -C rmmod clean
 	-rm -f *~ *.out hos.flp \#* hos_initrd hos_initrd.gz
 
 apps:
-	make -C apps
+	${MAKE} -C apps
 
 grub_image:
 	dd if=/dev/zero of=$(GRUB_IMAGE) bs=1024 seek=1440 count=0
@@ -75,7 +75,7 @@ initrd:
 	gzip -c $(INITRD) > $(INITRD).gz
 
 depend:
-	make -C kernel depend
+	${MAKE} -C kernel depend
 
 bochs:
 	bochs