At the point that we run this, nothing SDL-specific is set up yet.
__APPLE__ is a compiler predefined macro that forms part of the API on
Apple platforms, so it's fine to rely on it.
This partially reverts commit 31d133db.
Fixes: 31d133db "Define SDL_PLATFORM_* macros instead of underscored ones (#8875)"
Signed-off-by: Simon McVittie <smcv@collabora.com>