Scripting issues

Hi

I come from a MSSQL background and am trying to figure out how to write
deployment scripts for PostgreSQL. Typically, if I want to drop a
function, I would write a script that first checks for it's existence and
then performs the drop.

In MSSQL it would be as easy as (this can be done in native TSQL):
IF EXISTS (...some query to system tables...)
  DROP function XXX

However, I am really strugling to understand how to do this in PostgreSQL.
It seem the standard SQL language doesn't support the IF statement.

All the development that I do needs to be deployed in a script fashion and
generally I need to check for the existence of an object before replacing
or dropping.

Any help will be much appreciated.

Thanks
Craig