Long-term, this should be fixed by just requiring macOS to use GNU make (probably via setting some alias), but this change is really overdue and not having a macOS computer makes this harder to test.
Rationale: uninstalling devkitarm doesn't remove base_tools, but removes bin. If the binaries exist but not base_tools, then the user clearly wants to install using dkA. Otherwise, if base_tools exists but not binaries, it is possible that the user wants to use standalone arm-none-eabi binaries.