Proposed cleanup of generated header files

It's bothered me for some time that backend files need to be compiled
with -I src/backend as well as -I src/include. AFAICT this is just
because the two header files that are generated on-the-fly (parse.h
and fmgroids.h, formerly known as fmgr.h) are included from src/backend
rather than being inserted into the include tree, which it seems to me
is where they should be. Any objections if I rearrange the makefiles
so that these files get placed under include/ when they are built,
and then -I src/backend goes away?

(In case anyone is wondering, there are no platform-dependencies in
either file. We could distribute them as part of the distribution
tarball --- in fact we already do so for parse.h. So I don't see
that installing them into src/include would create any problems for
multiplatform builds.)

regards, tom lane