First of all, thank you for your help.
Second, your instructions at https://forum.audacityteam.org/t/building-audacity-2-3-1-on-ubuntu-18-04/52255/1 work fine, both on a fresh install of Ubuntu 19.04, and Pop!_OS 19.04.
In case this helps anyone else, building wxWidgets from source is neither necessary nor advisable. I ran into all kinds of trouble because of this. Even when I eventually did a sudo make uninstall in the wxWidgets build folder to get rid of my wxWidgets, it did not completely remove itself, and it made the Audacity configure script think wxWidgets headers were installed in /usr/local. To avoid deleting anything that might have been installed by your instructions, I ended up trying to install Pop!_OS inside Virtualbox, failing miserably, and finally successfully installing Ubuntu 19.04. Your instructions worked perfectly on a fresh install of Ubuntu 19.04. I then compared it to what I had on my host machine and manually deleted /usr/local/lib/wx and /usr/local/include/wx3.0 and everything under them; after doing that, the configure step was able to find everything in its proper place, and compilation finished with no problems.
Once again, if building on Ubuntu, everything you need is installed for you by the sudo apt-get build-dep audacity step in the instructions, so don’t try to build wxWidgets yourself.