Looks like the KallistiOS Dreamcast SDK disables double support
by default: 495e77fd60/environ_dreamcast.sh (L16)
We don't really need doubles in this code.
The one place where we might have needed them is the SMK video
decoder, handled in a separate PR.
This variable is controlled/set by the code in that file, this also lets other files have more specific/relevant includes instead of the monolith that is diablo.h