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

This returns something like "N-73264-gb54ac84". This is much more useful
than the individual library versions, of which there are too much and
which are very hard to map back to releases or git commits.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2014-08-09
 
 API changes, most recent first:
 
+2015-xx-xx - xxxxxx - lavu  53.xx.100
+  Add avutil_version_info().
+
 -------- 8< --------- FFmpeg 2.7 was cut here -------- 8< ---------
 
 2015-06-04 - cc17b43 - lswr  1.2.100