kbuild: macrofy the condition of if_changed and friends

Add a new macro that expands into $(newer-prereqs)$(cmd-check).

It makes it easier to add common code for if_changed, if_changed_dep,
and if_changed_rule.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Kbuild.include

@@ -130,13 +130,15 @@ make-cmd = $(call escsq,$(subst $(pound),$$(pound),$(subst $$,$$$$,$(cmd_$(1))))
 # PHONY targets skipped in both cases.
 newer-prereqs = $(filter-out $(PHONY),$?)
 
+if-changed-cond = $(newer-prereqs)$(cmd-check)
+
 # Execute command if command has changed or prerequisite(s) are updated.
-if_changed = $(if $(newer-prereqs)$(cmd-check),                              \
+if_changed = $(if $(if-changed-cond),                                        \
 	$(cmd);                                                              \
 	printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)
 
 # Execute the command and also postprocess generated .d dependencies file.
-if_changed_dep = $(if $(newer-prereqs)$(cmd-check),$(cmd_and_fixdep),@:)
+if_changed_dep = $(if $(if-changed-cond),$(cmd_and_fixdep),@:)
 
 cmd_and_fixdep =                                                             \
 	$(cmd);                                                              \
@@ -146,7 +148,7 @@ cmd_and_fixdep =                                                             \
 # Usage: $(call if_changed_rule,foo)
 # Will check if $(cmd_foo) or any of the prerequisites changed,
 # and if so will execute $(rule_foo).
-if_changed_rule = $(if $(newer-prereqs)$(cmd-check),$(rule_$(1)),@:)
+if_changed_rule = $(if $(if-changed-cond),$(rule_$(1)),@:)
 
 ###
 # why - tell why a target got built