Fixed mingw 4.8.1 gcc build.
Fixed mingw 4.8.1 gcc build.
Related changes: https://github.com/avplayer/avhttp/commit/d197676fa73ff65ea95a99b057c7a9e4a99879ab  and https://github.com/avplayer/avhttp/commit/6f4f55acc7763d904f2c0a70c69d9af3cd8754c0
Related issue: https://github.com/avplayer/avhttp/issues/10
ditto
use async_dir_walk_continue_handler
add async_foreach , and let async_dir_walk use async_foreach