Created by: stepheny
It is great to stop trying to build against static ffmpeg library early. But only showing "unknown flags" -pthread is very confusing.
This patch trys to check 'library_dirs' returned by parse_cflags.
If system ffmpeg was used, this should be empty. If a custom built ffmpeg was used, probably in /usr/local/lib, this would be filled by pkg-config. And then check if target .so file exists there. If it does not exist, the library is probably static. In case this test could go wrong, an env var was added to bypass the test.
I think this should work on linux. I've no idea how this would affect windows or macos users.
I've no idea whether the env var name and error message are proper.