In a few days the program will be able to decode flv files. That doesn’t mean it will be able to fully decode the video or audio content, but the stream encapsulation will be recognized.
Unfortunately, this not works for compile and install missing proprietary and free codecs from FFmpeg. If you compile any codec was missing, FFmpeg will not find the codec on compile.
I wrote some notes on compiling ffmpeg with third party codecs and libraries here. I also included some notes on finding, building, and installing the codec and other libraries so that the ffmpeg build script can find and include them. I hope these notes can help you.
There are ‘build info’ links on the page that point to lists of the third party libraries included and the codecs enabled in the respective builds offered.