Is it possible to use variables in httpd.conf

Try mod_macro. It actually allows you to use what are essentially variables. An example from the module's docs page gives the gist of it:

## Define a VHost Macro for repetitive configurations

<Macro VHost $host $port $dir>
  Listen $port
  <VirtualHost *:$port>

    ServerName $host
    DocumentRoot $dir

    <Directory $dir>
      # do something here...
    </Directory>

    # limit access to intranet subdir.
    <Directory $dir/intranet>
      Require ip 10.0.0.0/8
    </Directory>
  </VirtualHost>
</Macro>

## Use of VHost with different arguments.

Use VHost www.apache.org 80 /vhosts/apache/htdocs
Use VHost example.org 8080 /vhosts/example/htdocs
Use VHost www.example.fr 1234 /vhosts/example.fr/htdocs

I found a download for it at http://www.cri.ensmp.fr/~coelho/mod_macro/

Yes, kind of. You can get environment variables substituted in to the config file at start up with the ${ENVVAR} syntax. It's up to you to figure out how to set those variables before starting up the server.

http://httpd.apache.org/docs/2.2/configuring.html#syntax

Note that these variables will persist so any scripts in languages like php will be able to read them.

Also it's important to note that these values are interpreted once only when the server starts up so they're more like constants than variables.

Update

As of httpd version 2.4, see this answer instead: https://stackoverflow.com/a/15731921/498798