Fix ecpg dependency on postgres.h

I attached the patch which fix ecpg dependency on postgresql.h. This was
discussed on hackers:

http://archives.postgresql.org/pgsql-hackers/2007-10/msg01261.php

Directory pgsql/src/interfaces/ecpg/preproc/parser should be removed and
there is new file pgsql/src/interfaces/ecpg/preproc/parser.c which is
little bit modified version (only different headers are included) of
pgsql/src/backend/parser/parser.c.

Zdenek