fix: ac-os build — don't pass USE_SDL unless explicitly set
Makefile's ifdef USE_SDL triggers on any value including 0.
Now only passes USE_SDL=1 when USE_SDL env var is explicitly "1".
Fixes build failure in devcontainers without SDL3 headers.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>