I prefer to use /opt but any directory will do.

sudo apt install -y git build-essential cmake autoconf libtool libao-dev libfftw3-dev librtlsdr-dev
cd /opt
sudo git clone https://github.com/theori-io/nrsc5.git
cd nrsc5
sudo chown -R ${USER}:${USER} .
mkdir build
cd build
# cmake -DUSE_NEON=ON            # Use NEON instructions. [ARM, default=OFF]
# cmake -DUSE_SSE=ON             # Use SSSE3 instructions. [x86, default=OFF]
# cmake -DUSE_FAAD2=ON           # AAC decoding with FAAD2. [default=ON]
# cmake -DLIBRARY_DEBUG_LEVEL=1  # Debug logging level for libnrsc5. [default=5]
# cmake ..                       # All others
make
sudo make install
sudo ldconfig