do not rebuild static files all the time

scripts/Makefile.static

@@ -7,11 +7,27 @@ GZ = $(patsubst %,%.gz,$(SRCS))
 .PHONY: all
 all: $(ZSTD) $(BR) $(GZ)
 
-%.br: %
-	brotli $<
+define MAKETARGET
 
-%.zst: %
-	zstd -19 $<
+$(1).br: $(1).xxhash
+	brotli -f $(1)
+	touch $(1).br
 
-%.gz: %
-	zopfli $<
+$(1).zst: $(1).xxhash
+	zstd -f -19 $(1)
+	touch $(1).zst
+
+$(1).gz: $(1).xxhash
+	zopfli $(1)
+	touch $(1).gz
+
+# https://github.com/gohugoio/hugo/issues/10842
+$(1).xxhash: $(1)
+	current=$$$$(xxhsum $$< | cut -d' ' -f1); \
+	if [ ! -f $$< ] || [ "$$$$(cat $$@ 2>/dev/null)" != "$$$$current" ]; then \
+		echo "$$$$current" > $$@; \
+	fi
+
+endef
+
+$(foreach f,$(SRCS),$(eval $(call MAKETARGET,$(f))))