fix: disable SDL in oven builds — DRI/GBM segfault on some hardware
SDL3/GBM/Mesa DRI libs caused immediate segfault on ThinkPad 11e
(libgbm.so cannot open DRI driver). Build now defaults to DRM-only
(AC_BUILD_SDL=0). SDL can be re-enabled per-build with AC_BUILD_SDL=1.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>