Solved. The problem is with the Seg3D version. Although googling "Seg3D
download" takes you to the page

https://www.sci.utah.edu/download/seg3d

that states "(Latest Version 2.2.1)" and provides download links, this seems
to be an outdated page.

Instead, go to the Seg3D github releases page

https://github.com/SCIInstitute/Seg3D/releases

Download the tar.gz source code for v2.4.2, and unpack to ~/Software.

There are general instructions to build in linux here

http://sciinstitute.github.io/seg3d.pages/build.html

But they need some tweaking for Ubuntu.

At the very least you need the following ubuntu packages (g++ is v7.2.0 at the
time of this writing), and possibly some others that I had already in my
system

sudo apt install git subversion qt5-default libqt5svg5-dev g++ gcc

Then you can build Seg3D2 with

cd ~/Software/Seg3D-2.4.2
cd bin
cmake -DQt5_PATH:PATH=/usr/lib/x86_64-linux-gnu ../Superbuild/
make -j6

The binary that gets built is
~/Software/Seg3D-2.4.2/bin/Seg3D/Seg3D2
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