[FFmpeg-user] how to include V4L2_PIX_FMT_H264

[Soho Soho123]

hi all,

I see the code :
ffmpeg-951d39f/libavdevice/v4l2.c,
static struct fmt_map fmt_conversion_table[]

#ifdef V4L2_PIX_FMT_H264
    { AV_PIX_FMT_NONE,    AV_CODEC_ID_H264,     V4L2_PIX_FMT_H264    },
#endif

But I donot know how to enable "V4L2_PIX_FMT_H264"?
there is no any information about V4L2_PIX_FMT_H264 in
ffmpeg-951d39f/libavutil/pixfmt.h and old_pix_fmts.h.
Does anyone have idea?