[exim] Confusing Configuration

I googled and hunted through documentation for six hours trying to
figure out if exim4 was doing a reverse lookup or not, and how to turn
it on and off.

I see this line in 30_exim4-config_check_rcpt

  .ifdef CHECK_RCPT_REVERSE_DNS
   warn
     message = X-Host-Lookup-Failed: Reverse DNS lookup failed for 
$sender_host_$
      condition = ${if 
and{{def:sender_host_address}{!def:sender_host_name}}\
                       {yes}{no}}
   .endif

But I have no idea where one would define or undefine CHECK_RCPT_REVERSE_DNS

I also understand there is some magical way to "rebuild" the real conf
file from all the fake conf files called update-exim4.conf

I'm sure there must be some easy way to turn things on or off for exim4
without having to run through dpkg-reconfigure and have to guess at
answers to every single option.

any help?