[PATCH] Fix __XEN_INTERFACE_VERSION__ old config breakage

[Magnus Damm <magnus@valinux.co.jp>]

Fix __XEN_INTERFACE_VERSION__ old config breakage

For old kernel configurations that lack CONFIG_XEN_INTERFACE_VERSION,
__XEN_INTERFACE_VERSION__ will currently be defined but set to nothing. 

If a xen-enabled kernel is built separately from the xen-unstable dist,
the following error occurs with xen-unstable-9415:

#
# using defaults found in .config
#
  SPLIT   include/linux/autoconf.h -> include/config/*
  CC      arch/i386/kernel/asm-offsets.s
In file included from include/xen/interface/xen.h:439,
                 from include/asm/mach-xen/asm/page.h:20,
                 from include/asm/thread_info.h:14,
                 from include/linux/thread_info.h:21,
                 from include/linux/preempt.h:10,
                 from include/linux/spinlock.h:50,
                 from include/linux/capability.h:45,
                 from include/linux/sched.h:7,
                 from arch/i386/kernel/asm-offsets.c:7:
include/xen/interface/xen-compat.h:22:31: #if with no expression
include/xen/interface/xen-compat.h:26:31: #if with no expression
make[1]: *** [arch/i386/kernel/asm-offsets.s] Error 1
make: *** [prepare0] Error 2
$

The patch below fixes the problem and makes the logic in xen-compat.h work
correctly - __XEN_INTERFACE_VERSION__ now becomes unset if the config option
is missing.

Signed-Off-By: Magnus Damm <magnus@valinux.co.jp>

--- linux-2.6-xen-sparse/arch/i386/Makefile
+++ linux-2.6-xen-sparse/arch/i386/Makefile	2006-03-31 16:36:09.000000000 +0900
@@ -45,8 +45,10 @@ CFLAGS				+= $(shell if [ $(call cc-vers
 
 CFLAGS += $(cflags-y)
 
+ifneq   "$(CONFIG_XEN_INTERFACE_VERSION)" ""
 cppflags-$(CONFIG_XEN) += \
 	-D__XEN_INTERFACE_VERSION__=$(CONFIG_XEN_INTERFACE_VERSION)
+endif
 
 CPPFLAGS += $(cppflags-y)