You will need an IWAD for PrBoom to be fully functional.  An IWAD is
the main data file containing the graphics and levels for Doom.  If
you have a copy of one of the original Doom games, simply copy your
Doom, Doom 2, Ultimate Doom, or Final Doom IWAD (doom.wad, doom2.wad,
tnt.wad, and plutonia.wad respectively) to /usr/local/share/prboom/.
If you do not possess any of those, you can use the shareware IWAD
which is available at:

ftp://ftp.fu-berlin.de/pc/msdos/games/idgames/idstuff/doom/doom-1.8.wad.gz
SHA256 (doom-1.8.wad.gz) = WM9qVjtjGkdWFjCvpidthTwnB5NC95aewwBpZdqFV6E=

Install it to /usr/local/share/prboom as "doom.wad".
