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

This returns something like "N-73264-gb54ac840". 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

libavutil/avutil.h

@@ -170,6 +170,13 @@
  */
 unsigned avutil_version(void);
 
+/**
+ * Return an informative version string. This usually the actual release version
+ * number and a git hash. This string has no fixed format and can change any
+ * time. It should never be parsed by code.
+ */
+const char *avutil_version_info(void);
+
 /**
  * Return the libavutil build-time configuration.
  */

libavutil/utils.c

@@ -30,6 +30,11 @@
 #include "libavutil/ffversion.h"
 const char av_util_ffversion[] = "FFmpeg version " FFMPEG_VERSION;
 
+const char *avutil_version_info(void)
+{
+    return FFMPEG_VERSION;
+}
+
 unsigned avutil_version(void)
 {
     static int checks_done;