Add #ifndef around FF_API_MM_FLAGS.

The #ifndef is required to allow for example some automated regression
tests by simply configuring with: --extra-cflags="-DFF_API_FOO=0".

Originally committed as revision 25043 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -51,7 +51,9 @@
 #ifndef FF_API_PALETTE_CONTROL
 #define FF_API_PALETTE_CONTROL  (LIBAVCODEC_VERSION_MAJOR < 54)
 #endif
+#ifndef FF_API_MM_FLAGS
 #define FF_API_MM_FLAGS         (LIBAVCODEC_VERSION_MAJOR < 53)
+#endif
 
 #define AV_NOPTS_VALUE          INT64_C(0x8000000000000000)
 #define AV_TIME_BASE            1000000