Running CREATE only on certain Postgres versions

I have some code which creates a function in Postgres, taken from
http://wiki.postgresql.org/wiki/Array_agg .

DROP AGGREGATE IF EXISTS array_agg(anyelement);
CREATE AGGREGATE array_agg(anyelement) (
SFUNC=array_append,
STYPE=anyarray,
INITCOND='{}'
);

The function was added in 8.4, and so the code fails when run on 8.4 or higher.

How can I make the code cross-version compatible? For instance, how
can I tell it to check the version, and only run if 8.3 or lower?   Or
another way to make it cross-version?