I'm getting the same error, using g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
1.15.3 builds fine, naturally, and I can't find any differences in the main.cpp file between the two versions. Even the config-generated Makefile is virtually identical - and using the previous version's Makefile doesn't work either. I've attached my autoconf-generated config.log file in the hopes that it might be useful.
I wanted to add that I'm trying to recompile because I recently upgraded to 20.04. After the upgrade, makemkv can't find libQtGui.so.4. After trying to find packages that supply it failed, I went to do the upgrade to 1.15.4. I don't have older copies to compare, so I can't help there.