This 'hpcdtoppm' directory is really just a placeholder for the real
Hpcdtoppm source code.  

The real Hpcdtoppm source code cannot be distributed on Sourceforge
because a copyright holder does not license it in a way open enough to
meet Sourceforge's requirements.

Therefore, the Netpbm maintainer distributes Hpcdtoppm via another channel
and distributes this dummy directory in the main Netpbm source tree on 
Sourceforge.  When you run the program named 'hpcdtoppm' in this directory,
it tells you how to get the real Hpcdtoppm.

When you get the real Hpcdtoppm tarball, just unpack it and replace
this entire 'hpcdtoppm' directory with its contents.  Then build
Netpbm normally.

Bear in mind when you get the real Hpcdtoppm, that it is not as openly
licensed as what's in the rest of the Netpbm source tree.  As you'll
see in the license that comes with Hpcdtoppm, you may _not_ sell it to
someone else.)