ffmpeg: detect which ffmpeg headers should be included

Since ffmpeg svn r12865, you have to include libavcodec/avcodec.h
instead of avcodec.h.  This cannot be checked at compile time, instead
we have to add a check to configure.ac.  Viliam's original ffmpeg
plugin was based on the newer ffmpeg library, while my Debian
installation had the older version.  My attempt to correct his include
statements wasn't correct after all.

src/inputPlugins/ffmpeg_plugin.c

@@ -29,9 +29,15 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#ifdef OLD_FFMPEG_INCLUDES
 #include <avcodec.h>
 #include <avformat.h>
 #include <avio.h>
+#else
+#include <libavcodec/avcodec.h>
+#include <libavformat/avformat.h>
+#include <libavformat/avio.h>
+#endif
 
 typedef struct {
 	int audioStream;