Set paths:
SuperCollider SC_PATH and SuperCollider Extensions path
export SC_PATH=/usr/include/SuperCollider/
export DYNLIB_CLASS_PATH=$HOME'/.local/shareSuperCollider/Extensions/Dynlib/classes/'
export DYNLIB_PLUGIN_PATH=$HOME'/.local/share/SuperCollider/Extensions/Dynlib/plugins/'
Building
cd src
./build_and_cp.sh
g++ -fpic -c DemoSin.cpp -o DemoSin.o -Ofast
gcc -fpic -shared DemoSin.o -lm -o libDemoSin.so -Ofast