lavu: add an API function to return the Libav version string

This returns something like "v12_dev0-1332-g333a27c". This is much more
useful than the individual library versions, of which there are too
many, and which are very hard to map back to releases or git commits.

Signed-off-by: Janne Grunau <janne-libav@jannau.net>

libavutil/avutil.h

@@ -163,6 +163,13 @@
  */
 unsigned avutil_version(void);
 
+/**
+ * Return an informative version string. This usually is the actual release
+ * version number or a git commit description. This string has no fixed format
+ * and can change any time. It should never be parsed by code.
+ */
+const char *av_version_info(void);
+
 /**
  * Return the libavutil build-time configuration.
  */