Using environment variable to store capture options (#9292)

* Using environment variable to store options parsed by av_dict_parse_string(ENV{OPENCV_FFMPEG_CAPTURE_OPTIONS}, ";", "|")

* Adding missing mandatory flags parameter

* Guarding against missing function via LIBAVUTIL version

* Code review fixes

Copy/paste error due to coder mistake reverted
Proper version checking for LIBAVUTIL_BUILD

modules/videoio/src/cap_ffmpeg_impl.hpp

@@ -781,7 +781,23 @@ bool CvCapture_FFMPEG::open( const char* _filename )
 #endif
 
 #if LIBAVFORMAT_BUILD >= CALC_FFMPEG_VERSION(52, 111, 0)
+#ifndef NO_GETENV
+    char* options = getenv("OPENCV_FFMPEG_CAPTURE_OPTIONS");
+    if(options == NULL)
+    {
+        av_dict_set(&dict, "rtsp_transport", "tcp", 0);
+    }
+    else
+    {
+#if LIBAVUTIL_BUILD >= (LIBAVUTIL_VERSION_MICRO >= 100 ? CALC_FFMPEG_VERSION(52, 17, 100) : CALC_FFMPEG_VERSION(52, 7, 0))
+        av_dict_parse_string(&dict, options, ";", "|", 0);
+#else
+        av_dict_set(&dict, "rtsp_transport", "tcp", 0);
+#endif
+    }
+#else
     av_dict_set(&dict, "rtsp_transport", "tcp", 0);
+#endif
     int err = avformat_open_input(&ic, _filename, NULL, &dict);
 #else
     int err = av_open_input_file(&ic, _filename, NULL, 0, NULL);