[PATCH] build: tweak variable exporting for make 3.82

[Jan Beulich <jbeulich@suse.com>]

While I've been running into an issue here only because of an additional
local change I'm carrying, to be able to override just the compiler in
$(XEN_ROOT)/.config (rather than the whole tool chain), in
config/StdGNU.mk:

ifeq ($(filter-out default undefined,$(origin CC)),)

I'd like to propose to nevertheless correct the underlying issue:
Exporting an unset variable changes its origin from "undefined" to
"file". This comes into effect because of our adding of -rR to
MAKEFLAGS, which make 3.82 wrongly applies also upon re-invoking itself
after having updated auto.conf{,.cmd}.

Move the export statement past $(XEN_ROOT)/config/$(XEN_OS).mk inclusion
such that the variables already have their designated values at that
point, while retaining their initial origin up to the point they get
defined.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/Makefile
+++ b/xen/Makefile
@@ -17,8 +17,6 @@ export XEN_BUILD_HOST	?= $(shell hostnam
 PYTHON_INTERPRETER	:= $(word 1,$(shell which python3 python python2 2>/dev/null) python)
 export PYTHON		?= $(PYTHON_INTERPRETER)
 
-export CC CXX LD
-
 export BASEDIR := $(CURDIR)
 export XEN_ROOT := $(BASEDIR)/..
 
@@ -42,6 +40,8 @@ export TARGET_ARCH     := $(shell echo $
 # Allow someone to change their config file
 export KCONFIG_CONFIG ?= .config
 
+export CC CXX LD
+
 .PHONY: default
 default: build