change installation directory of postgresql7

[A. Nooitgedagt]

What does it take to make port databases/postgresql7 
install in subdirectory /usr/local/pgsql ? The port's
default is /usr/local but postgresql's default is /usr/local/pgsql.

Is there another way than changing PREFIX in the port?

It doesn't seem to matter if I define
CONFIGURE_ARGS+= --prefix=${PREFIX}/pgsql

I think I have tried most other possibilities,
but I'm probably missing something

Thx

Aldert