avformat_find_stream_info was introduced in Libav avformat 53.3,
but it was only merged to FFmpeg in avformat 53.6.

In Libav avformat 53.3-53.5 av_find_stream_info is not removed
(only deprecated), so this shouldn't break building with that.

at can't be a null pointer, so the condition was always false, and
a nonsensical pattern like "image.png%00d" was being inferred.

CMake's get_filename_component with NAME_WE parameters, interprets first period as start of extension. For. e.g file name with out extension of 'this.is.a.text.file.txt' will be taken as 'this'. Hence using NAME with regex replacement to get 'this.is.a.text.file'

Also, I made a separate macro for the current function name, which
helps simplify a lot of code that uses it.