ffmpeg: Enforce minimum supported versions of ffmpeg

lavcodec:  54.86.100
lavformat: 54.59.106
lavutil:   52.13.100

These are the versions included in FFmpeg 1.1, which is the oldest
release that works with Panda already: we've been using
`av_opt_set_sample_fmt` (introduced in FFmpeg 1.1) since
03e96d8c4a903be7222365b40613429768659892 (August 2013) and nobody has
complained since. In other words, I'm not dropping support for anything
here, I'm just making the supported versions explicit.

panda/src/ffmpeg/config_ffmpeg.cxx

@@ -22,12 +22,26 @@
 
 extern "C" {
   #include "libavcodec/avcodec.h"
+  #include "libavformat/avformat.h"
+  #include "libavutil/avutil.h"
 }
 
 #if !defined(CPPPARSER) && !defined(BUILDING_FFMPEG)
   #error Buildsystem error: BUILDING_FFMPEG not defined
 #endif
 
+#if LIBAVCODEC_VERSION_INT < AV_VERSION_INT(54, 86, 100)
+  #error Minimum supported version of libavcodec is 54.86.100.
+#endif
+
+#if LIBAVFORMAT_VERSION_INT < AV_VERSION_INT(54, 59, 106)
+  #error Minimum supported version of libavformat is 54.59.106.
+#endif
+
+#if LIBAVUTIL_VERSION_INT < AV_VERSION_INT(52, 13, 100)
+  #error Minimum supported version of libavutil is 52.13.100.
+#endif
+
 ConfigureDef(config_ffmpeg);
 NotifyCategoryDef(ffmpeg, "");