Needs to update revision.h unless existing [ci skip]

defs/gmake.mk

@@ -338,8 +338,13 @@ $(UNICODE_SRC_DATA_DIR)/.unicode-tables.time: \
 	$(UNICODE_FILES) $(UNICODE_PROPERTY_FILES)
 endif
 
+ifeq ($(wildcard $(srcdir)/revision.h),)
+REVISION_IN_HEADER := none
+REVISION_LATEST := update
+else
 REVISION_IN_HEADER := $(shell sed -n 's/^\#define RUBY_FULL_REVISION "\(.*\)"/\1/p' $(srcdir)/revision.h 2>/dev/null)
 REVISION_LATEST := $(shell $(CHDIR) $(srcdir) && git log -1 --format=%H 2>/dev/null)
+endif
 ifneq ($(REVISION_IN_HEADER),$(REVISION_LATEST))
 # GNU make treat the target as unmodified when its dependents get
 # updated but it is not updated, while others may not.