explain how to test it

Originally committed as revision 8352 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/sha1.c

@@ -118,6 +118,8 @@ void av_sha1_final(AVSHA1* context, uint8_t digest[20]){
     }
 }
 
+// use the following to test
+// gcc -DTEST -DHAVE_AV_CONFIG_H -I.. sha1.c -O3 -W -Wall -o sha1 && time ./sha1
 #ifdef TEST
 #include <stdio.h>
 #undef printf