Caution: the compile/install process is very weird.
These notes helped me use it on a Debian system:

> Each time I re-install a fresh Linux distribution, one of the first things I need to do is to compile the Heirloom Documentation Tools.
> However some adjustments have to be performed; I write them here in order to recall them later:
>
>   * download the sources from http://heirloom.sourceforge.net/doctools.html
>   * type: `sudo apt-get install g++ libc-dev bison flex`
>   * uncompress the tarball
>   * enter the `heirloom-doctools` directory
>   * edit `mk.config` and change three lines:
>
>     - replace `INSTALL=/usr/ucb/install` with `INSTALL=/usr/bin/install`
>     - replace `PREFIX=/usr` with `PREFIX=/opt/heirloom`
>     - replace `SHELL=/sbin/sh` with `SHELL=/bin/sh`
>
>   * run `make`
>   * run `sudo make install`
>
> This will perform a clean install into `/opt/heirloom` and you have to fix your `PATH` accordingly.

Source: <https://gist.github.com/baruchel/0653cc7c041d58d21b202c8e94e4ea30>