Warn on guaranteed useless lines

[Dan Jacobson]

Perhaps on (info "(make) Multiple Targets"), etc.
mention why the :%.bla lines have no effect,

$ cat Makefile
%.fetch:; echo Fetched
%.install:%.bla
%.install:%.fetch; echo Installed
%.install:%.bla
%.bla:; echo haha
m.install:

nor produce warnings.