lavu/parseutils: add av_small_strptime()

Make internal small_strptime() function public, and use it in place of
strptime().
This allows to avoid a dependency on strptime() on systems which do not
support it.

In particular, fix trac ticket #992.

doc/APIchanges

@@ -15,6 +15,12 @@ libavutil:     2011-04-18
 
 API changes, most recent first:
 
+2012-09-01 - xxxxxxx - lavu 51.72.100 - parseutils.h
+  Add av_small_strptime() time parsing function.
+
+  Can be used as a stripped-down replacement for strptime(), on
+  systems which do not support it.
+
 2012-08-13 - xxxxxxx - lavfi 3.8.100 - avfilter.h
   Add avfilter_get_class() function, and priv_class field to AVFilter
   struct.