1. Build TuxPaint.app on Intel macOS, rename it TuxPaint-x86_64.app.
2. Build TuxPaint.app on Apple Silicon macOS, rename it TuxPaint-arm64.app.
3. Copy TuxPaint-x86_64.app to the same directory as TuxPaint-arm64.app
on the Apple Silicon macOS, then run:
macos/build-universal.sh TuxPaint-x86_64.app TuxPaint-arm64.app
... to produce TuxPaint.app that's universal.
4. `make TuxPaint.dmg` to produce TuxPaint.dmg from the universal
TuxPaint.app.