build scripts: support for separate build and source directories

kernel/Makefile.build.in

@@ -1,3 +1,5 @@
+VPATH=@abs_srcdir@
+SRC=$(VPATH)
 IHKDIR=$(IHKBASE)/$(TARGETDIR)
 OBJS = init.o mem.o debug.o mikc.o listeners.o ap.o syscall.o cls.o host.o
 OBJS += process.o copy.o waitq.o futex.o timer.o plist.o fileobj.o shmobj.o

kernel/Makefile.in

@@ -1,8 +1,8 @@
 BUILD_TARGET ?= @TARGET@
 KERNDIR=@KERNDIR@
-SRC = $(CURDIR)
+VPATH=@abs_srcdir@
 
-IHKBASE ?= $(SRC)/../../ihk/cokernel
+IHKBASE ?= $(VPATH)/../../ihk/cokernel
 O ?= $(CURDIR)/build
 V ?= $(VERBOSE)