[exim] Conditional file include?

All,

I'm trying to do a conditional file include so that I can roll out the
same exim configs to several hosts. I can't seem to get this to work:

    .include /etc/exim/host/${primary_hostname}.conf

    ---
    Starting exim: 2007-08-24 14:10:54 Exim configuration
    error in line 5 of /etc/exim/exim.conf: failed to open
    included configuration file
    /etc/exim/host/${primary_hostname}.conf
    ---

Can this be done? Does string replacement happen before include? If
not, is there some type of if/then/else or switch syntax that could be
cooked up to do different includes based on the value in $primary_hostname?

-- Dante