build: define "move-if-changed" make macro.

Use it to replace various places which (should) use the
	if ! cmp -s ...; then mv ....; fi
pattern.

Also add an else clause to cleanup the unchanged temporary file.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>

Config.mk

@@ -118,6 +118,16 @@ define buildmakevars2shellvars
     export XEN_ROOT="$(XEN_ROOT)"
 endef
 
+#
+# Compare $(1) and $(2) and replace $(2) with $(1) if they differ
+#
+# Typically $(1) is a newly generated file and $(2) is the target file
+# being regenerated. This prevents changing the timestamp of $(2) only
+# due to being auto regenereated with the same contents.
+define move-if-changed
+	if ! cmp -s $(1) $(2); then mv -f $(1) $(2); else rm -f $(1); fi
+endef
+
 buildmakevars2file = $(eval $(call buildmakevars2file-closure,$(1)))
 define buildmakevars2file-closure
     .PHONY: genpath
@@ -134,7 +144,7 @@ define buildmakevars2file-closure
 	echo "XEN_SCRIPT_DIR=\"$(XEN_SCRIPT_DIR)\"" >> $(1).tmp;           \
 	echo "XEN_LOCK_DIR=\"$(XEN_LOCK_DIR)\"" >> $(1).tmp;               \
 	echo "XEN_RUN_DIR=\"$(XEN_RUN_DIR)\"" >> $(1).tmp;                 \
-	if ! cmp $(1).tmp $(1); then mv -f $(1).tmp $(1); fi
+	$(call move-if-changed,$(1).tmp,$(1))
 endef
 
 ifeq ($(debug),y)

tools/libxl/Makefile

@@ -75,7 +75,8 @@ $(eval $(genpath-target))
 
 _libxl_paths.h: genpath
 	sed -e "s/\([^=]*\)=\(.*\)/#define \1 \2/g" $@.tmp >$@.2.tmp
-	if ! cmp -s $@.2.tmp $@; then mv -f $@.2.tmp $@; fi
+	rm -f $@.tmp
+	$(call move-if-changed,$@.2.tmp,$@)
 
 libxl_paths.c: _libxl_paths.h
 
@@ -85,8 +86,8 @@ $(LIBXL_OBJS) $(LIBXLU_OBJS) $(XL_OBJS): libxl.h
 
 _libxl_%.h _libxl_%.c: libxl.idl gen%.py libxl%.py
 	$(PYTHON) gen$*.py libxl.idl __libxl_$*.h __libxl_$*.c
-	mv __libxl_$*.h _libxl_$*.h
-	mv __libxl_$*.c _libxl_$*.c
+	$(call move-if-changed,__libxl_$*.h,_libxl_$*.h)
+	$(call move-if-changed,__libxl_$*.c,_libxl_$*.c)
 
 libxenlight.so: libxenlight.so.$(MAJOR)
 	ln -sf $< $@