build: check if replaygain side data is available

Signed-off-by: wm4 <wm4@nowhere>

old-configure change by wm4.
demux: add replaygain_data field to demuxer_t
af: add replaygain_data field to af_stream and af_instance

Closes #664
af_volume: use replaygain side data
demux: move metadata-based replaygain decoding out of af_volume
build: add -Wempty-body to compiler flags

Warns against "if(0);" but not "if(0){}" - perfect for our purposes.