Implement automatic FFmpeg version detection

Based on ffmpeg-sys-next 4.3.2.

src/format/format/flag.rs

@@ -6,7 +6,7 @@ bitflags! {
         const NO_FILE       = AVFMT_NOFILE;
         const NEED_NUMBER   = AVFMT_NEEDNUMBER;
         const SHOW_IDS      = AVFMT_SHOW_IDS;
-        #[cfg(not(feature = "ffmpeg4"))]
+        #[cfg(not(feature = "ffmpeg_4_0"))]
         const RAW_PICTURE   = AVFMT_RAWPICTURE;
         const GLOBAL_HEADER = AVFMT_GLOBALHEADER;
         const NO_TIMESTAMPS = AVFMT_NOTIMESTAMPS;