Basically you have to make sure you have all the supporting packages loaded via apt-get, build osmid as before, and install the binaries m2o and o2m to native/osmid, have erlang installed (I used erlang-base on the Pi) and build the osc.erl and pi_server.erl files to their equivalent beam files in the erlang folder. As far as scsynth and the plugins go these were picked up automatically by installing supercollider-server and sc3-plugins.
for the rest, create a short cut to the ruby binary in server/native/ruby/bin
I used (from the qt folder):
ln -s `which ruby` ../../server/native/ruby/bin
to do this.
#now for some stuff using cmake
#I think I had to `gem install aubio` first on RPi too
./unix-prebuild.sh #builds the documentation
./unix-config.sh #sets up the build folder
cmake --build .
when this has finished you should have the new sonic-pi binary in the newly created build folder
You can run it from there, or from <SONIC-PI_SRC>/bin/sonic-pi
You may have problems with the prebuild script. It deletes an existing ruby-aubio-prerelease gem from the vendor folder. (subsequent runs will show an error which you ignore. However I found I needed the gem aubio install (It was version 0.3.3). You also need as part of the initial packages installed libaubio5 (on debian)
NB at time of writing the latest commit to Sonic Pi is #3e587fc. There is a bug in the earlier commit Merge pull request #2234 from cmaughan/feature/threaded_analyzer
I have submitted a PR Update scope.h syntax #2237 to cure this, but you will have to carry out the changes yourself manually if you build right now.
Hope these rough notes are helpful.