Previously, these were declared in postgres_ext.h, but they are not
needed nearly so widely as the OID declarations, so that doesn't
necessarily make sense. Also, because postgres_ext.h is included
before most of c.h has been processed, the previous location creates
some problems for a pending patch.
Patch by me, reviewed by Dilip Kumar.
Discussion: http://postgr.es/m/CA+TgmoYc8oevMqRokZQ4y_6aRn-7XQny1JBr5DyWR_jiFtONHw@mail.gmail.com