OPT_BASE_STRUCT defines which struct the OPT_ macros (like OPT_INT etc.)
reference implicitly, since these macros take struct member names but no
struct type. Normally, only cfg-mplayer.h should need this, and other
places shouldn't be bothered with having to #undef it.
(Some files, like demux_lavf.c, still store their options in MPOpts. In
the long term, this should be removed, and handled like e.g. with VO
suboptions instead.)