Sigh, MSVC build broken again

I wrote:

It looks like the MSVC build scripts are cheating by not copying
parser/gram.h into the include tree where it should be. I'm not
entirely sure why that has worked at all in the past, but it's
not working any more.

On further consideration, it's probably not a good idea to let
plpgsql depend on gram.h anyway --- in the past we've taken measures
to confine the use of that file as narrowly as possible. I did a
bit of code refactoring instead.

regards, tom lane