Here is a branch in my darktable fork starting with darktable.org 4.2 release tag. It should automatically pull rawspeed from the correct fork, supporting OM System and OM-1 camera. (WB presets and noise profiles for OM-1 are added to JSON files within the main darktable tree.)
To get the source code:
git clone https://github.com/sarunasb/darktable.git
cd darktable
git checkout 4.2-om
git submodule init
git submodule update
To compile on Ubuntu I do:
mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/opt/darktable/ ..
make -j
sudo make install
Running /opt/darktable/bin/darktable --version should give:
this is darktable 4.2.om1
…
In case your previous darktable version was earlier than 4.2, then running this will upgrade database and possibly config. One can run this newly compiled 4.2.om1 something like this to preserve the existing DB and config:
/opt/darktable/bin/darktable --configdir "~/.config/darktable-test"