move NULL_IF_CONFIG_SMALL() definition into internal header

Originally committed as revision 16619 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/internal.h

@@ -302,4 +302,15 @@ static av_always_inline av_const float truncf(float x)
 }
 #endif /* HAVE_TRUNCF */
 
+/**
+ * Returns NULL if CONFIG_SMALL is true otherwise the argument
+ * without modifications, used to disable the definition of strings
+ * (for example AVCodec long_names).
+ */
+#if CONFIG_SMALL
+#   define NULL_IF_CONFIG_SMALL(x) NULL
+#else
+#   define NULL_IF_CONFIG_SMALL(x) x
+#endif
+
 #endif /* AVUTIL_INTERNAL_H */