OK OK here we go whoop whoop, finally some success, still testing, but at least we get to see the Atari screen running on the Pi. Just in case others or the author would like to know how to get this to work on the latest RPi4 running Pi OS 64-bit, where we go: -
sudo apt-get install libsdl1.2-dev libgtk2.0-dev --no-install-recommends
tar xzf atari++.tgz
./configure
make
sudo make install
then to actually run it: -
atari++ -FrontEnd SDL -Sound SDL
From the looks of it, the issues I've been experiencing could be related to building on the 64-bit version of Pi OS, so when I get a bit of time later I'll trial a build on the Pi OS 32-bit.