build: enable compiler optimization by default
player: don't wait forever with --cache-pause-below behavior

Commit dc00b146, which disables polling by default, missed another
instance of polling: when the player pauses automatically on low cache.

This could lead to apparent freezes when playing network streams.
demux: make the cache refresh cached STREAM_CTRLs

This fixes the same symptom as the previous commit, but when the demuxer
thread is enabled. In this case, if nothing was read from the demuxer,
the STREAM_CTRLs weren't updated either. To the player, this looked like
the stream cache was never making progress, so playback was kept paused.