[exim] include directive

I'm trying to use the .include (in fact the .include_if_exists)
directive and I'm not being successful at getting the filename to
include a string expansion.

What I want to do is:

.include_if_exists /var/exim/local/$primary_hostname.rc

and what I see in an strace is

7658 read(3, "\n#\n# If you are editing this fil"..., 4096) = 4096
7658 stat64("/var/exim/local/$primary_hostname.rc", 0xbfdd6300) = -1
ENOENT (No such file or directory)

So I'm guessing that string expansion isn't possible in an include
directive.

Jon.