build: add make macro for making file from file.in

Add a new make macro for creating <file> from <file>.in at build
time. To be used like this:

$(foreach file,$(IN_TARGETS),$(eval $(call apply-build-vars,$(file))))

This can be used instead of the current approach to perform the similar
step for file.in during ./configure.

This will avoid having to run ./configure just because of modifying a
file depending on a variable set by configure.

Prepare to have multiple files as source for the replacement patterns.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

Config.mk

@@ -159,6 +159,19 @@ define move-if-changed
 	if ! cmp -s $(1) $(2); then mv -f $(1) $(2); else rm -f $(1); fi
 endef
 
+PATH_FILES := Paths.mk
+INC_FILES = $(foreach f, $(PATH_FILES), $(XEN_ROOT)/config/$(f))
+
+-include $(INC_FILES)
+
+BUILD_MAKE_VARS = $(foreach f, $(PATH_FILES), $(shell awk '$$2 == ":=" { print $$1; }' $(XEN_ROOT)/config/$(f).in))
+
+# Replace @xxx@ markers in $(1).in with $(xxx) variable contents, write to $(1)
+define apply-build-vars
+ $(1): $(1).in $$(INC_FILES)
+	sed $$(foreach v, $$(BUILD_MAKE_VARS), -e 's#@$$(v)@#$$($$(v))#g') <$$< >$$@
+endef
+
 CFLAGS += -fno-strict-aliasing
 
 CFLAGS += -std=gnu99