Change built-in default privileges for CREATE DATABASE?

Hello,

when i CREATE a new DATABASE, its privileges column is empty (it's not
the same case as it being '{}', which means no privileges). Empty
column means: use built-in defaults. These defaults are:

=c + owner=CTc

How can I change these defaults so that after creation PUBLIC gets no
rights? That is: remove '=c' from them.

Here's a more detailed post on dba.SE:

http://dba.stackexchange.com/questions/43387/change-built-in-default-privileges-in-postgresql

Thanks,
Michał.