add a ff_gcd() function again, for compatibility with old libavcodec

Originally committed as revision 16814 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/mathematics.c

@@ -24,6 +24,7 @@
  */
 
 #include <assert.h>
+#include "avutil.h"
 #include "common.h"
 #include "mathematics.h"
 
@@ -54,6 +55,12 @@ int64_t av_gcd(int64_t a, int64_t b){
     else  return a;
 }
 
+#if LIBAVUTIL_VERSION_MAJOR < 50
+int64_t ff_gcd(int64_t a, int64_t b){
+    return av_gcd(a, b);
+}
+#endif
+
 int64_t av_rescale_rnd(int64_t a, int64_t b, int64_t c, enum AVRounding rnd){
     int64_t r=0;
     assert(c > 0);