On my EndeavourOS system, cmake couldn't find gtksourceviewmm-3.0 because gtksourceviewmm-4.0 was installed. I manually compiled and installed -3.0 alongside -4.0, bypassing the package manager, but that's a pretty ugly hack. It looks like the CMakeLists.txt file needs to be updated, but I couldn't figure out how to do that manually on my end.