prevent bitstream reader to be overriden

Originally committed as revision 6092 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/bitstream.h

@@ -8,10 +8,11 @@
 
 //#define ALT_BITSTREAM_WRITER
 //#define ALIGNED_BITSTREAM_WRITER
-
+#if !defined(LIBMPEG2_BITSTREAM_READER) && !defined(A32_BITSTREAM_READER) && !defined(ALT_BITSTREAM_READER)
 #define ALT_BITSTREAM_READER
 //#define LIBMPEG2_BITSTREAM_READER
 //#define A32_BITSTREAM_READER
+#endif
 #define LIBMPEG2_BITSTREAM_READER_HACK //add BERO
 
 extern const uint8_t ff_reverse[256];