SDL 2.0.5 dropped support for macOS 10.5 so target 10.6 instead. The
PPC build uses SDL 2.0.1 so it still targets 10.5. macOS 10.5 (x86,
x86_64) should automatically run the PPC build using Rosetta.
Revert MAN-AT-ARMS' change to SDL 2.0.8 SDL_platform.h that allowed
targeting macOS 10.5 for the sake of PPC. It also incorrectly allowed
x86 and x86_64 to target 10.5 as well. (Also macOS PPC uses separate
headers now.)