Two mods already: Denton's Enhanced Doom3 and Classic Doom 3!
Currently this needs my mod-support branch:
https://github.com/DanielGibson/dhewm3/tree/mod-support
Clone dhewm3, change to the mod-support branch and pull
git clone https://github.com/DanielGibson/dhewm3.git
cd dhewm3
git checkout mod-support
Then this (dhewm3-mods) repository must be cloned to /path/to/your-dhewm3/neo/mods/
That's done like this:
cd /path/to/your-dhewm3/neo/
git clone --recurse-submodules https://github.com/DanielGibson/dhewm3-mods.git mods
--recurse-submodules
will make sure that the submodules (containing the mods) will
be cloned as well.
Then just create your dhewm3 Makefiles with something like:
cd /path/to/your-dhewm3/
mkdir build-with-mods
cd build-with-mods
cmake -DMODS=ON ../neo/
and compile as usual.
See the dhewm3 README
for details on compiling (dependencies etc).
The mod directories contain a README.txt that will tell you where to get the mods data (levels etc) and possibly how to install it.
The actual mods lie in the dhewm3-base-mods repository, one branch per mod (the master branch is just the vanilla base game code from dhewm3) and are "linked" as git submodules.
dhewm3's cmake (at least in the aforementioned branch) adds the CMakeLists.txt of this repository and the CMakeLists.txt adds all the linked mod subdirectories (which themselves have their own CMakeLists.txt).
https://github.com/DanielGibson/dhewm3-base-mods#readme has information how to add additional mods.
Please note that the mods sourcecode must be available for that - and to be
included in this repository it must be under GPLv3 (that's the license
id Software used to release the Doom3 Source) - and not just Doom3 SDK License.
That's because mixing GPL code and SDK Licensed code is illegal.
This means that the mod authors/coders need to agree to publish their source under GPLv3.
If you are a mod author and want your mod in this list (and thus making it playable on all platforms supported by dhewm3 and possibly other Doom3 ports), but can't port it to dhewm3 yourself (or don't have the time or whatever), get in touch, maybe I can help you :-)
Cheers,
-Daniel