This is the game code part of OpenArena. In mod form it is referred as OpenArenaExpanded (OAX).
You need a C-compiler (tested with gcc and clang) and GNU make then just type
make
and the qvm-files will be build. Ready to be packed into a pk3-file.
See https://github.com/OpenArena/gamecode/wiki/Build-instruction for more details.
See http://openarena.wikia.com/wiki/OpenArena_eXpanded for alternative build options
It is possible to extract entity definition for use with GtkRadiant and NetRadiant like this:
cd code/game
./extract_entities.sh > openarena.def
Development documentation is located here: https://github.com/OpenArena/gamecode/wiki
The development board on the OpenArena forum: http://openarena.ws/board/index.php?board=30.0
In particular the Open Arena Expanded topic: http://openarena.ws/board/index.php?topic=1908.0
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.