Make build variable change aware

.gitignore

@@ -1,2 +1,3 @@
 # Generated with m4
 version.go
+.version*

Makefile

@@ -18,7 +18,12 @@ all: $(LIBRARY)
 $(LIBRARY): $(SOURCES)
 	go install $(PACKAGE)
 
-version.go: version.go.m4
+.version:
+	-@printf "%s\n%s\n%s" \
+		"$(VERSION)" "$(REVISION)" "$(BUILDTIME)" >$@.new
+	-@diff $@ $@.new >/dev/null 2>&1 && rm $@.new || mv $@.new $@
+
+version.go: version.go.m4 .version
 	-@m4 -Dm4_version=$(VERSION) \
 	   -Dm4_revision=$(REVISION) \
 	   -Dm4_build_time=$(BUILDTIME) \
@@ -26,4 +31,5 @@ version.go: version.go.m4
 
 clean:
 	-@rm -f version.go 2>/dev/null
+	-@rm -f .version 2>/dev/null
 	-@rm -f $(LIBRARY) 2>/dev/null