Makefile for parser

I've started doing a bit of work on gram.y, and am noticing some new
cruftiness in the Makefile: if I add tokens to gram.y/keywords.c then I
can't just remake in that directory since parse.h is not updated
elsewhere in the tree.

I believe that the Makefile used to reach up and over in the tree to
update parse.h, and I'll guess that this fell victim to a general
cleanup (looks like Tom Lane and Peter E. have been working with the
Makefiles, but I haven't tracked down the details).

Any suggestions for a fixup? In general, I agree that having Makefiles
muck with stuff elsewhere in the tree is a Bad Idea, but it would be
nice if each directory could be built on its own.

- Thomas