Add 'make' target 'build master' to remember what to update when buil…

[lefessan]

…ding

after a long while

[nberth]

I guess you will likely be the only user of this rather risky rule ;)

[nberth]

Wondering: wouldn't a make build-help rule be more meaningful, if it's to remember what to do:
Actually something with

.PHONY: build-help
build-help:
	$(MAKE) -n build-master

(.PHONY: build-master is missing as well, but not that critical.)