Install prerequisites

sudo apt install -y libfltk1.3-dev libpng-dev libsamplerate0-dev libsndfile1-dev portaudio19-dev

Download the FLDigi and FLRig source

mv ~/Downloads/flrig* ~/src
mv ~/Downloads/fldigi* ~/src
tar -zxvf fldigi*
tar -zxvf flrig*
cd fldigi*
./configure
make -j<numprocs>
sudo make install
cd ..
cd flrig*
./configure
make -j<numprocs>
sudo make install