Make demuxers auto-select parsers they need

This makes configure --disable-everything --enable-demuxer=foo
work as expected.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -1352,19 +1352,19 @@ libxavs_encoder_deps="libxavs"
 libxvid_encoder_deps="libxvid"
 
 # demuxers / muxers
-ac3_demuxer_deps="ac3_parser"
+ac3_demuxer_select="ac3_parser"
 asf_stream_muxer_select="asf_muxer"
 avisynth_demuxer_deps="avisynth"
-dirac_demuxer_deps="dirac_parser"
+dirac_demuxer_select="dirac_parser"
 eac3_demuxer_select="ac3_parser"
-flac_demuxer_deps="flac_parser"
+flac_demuxer_select="flac_parser"
 ipod_muxer_select="mov_muxer"
 libnut_demuxer_deps="libnut"
 libnut_muxer_deps="libnut"
 matroska_audio_muxer_select="matroska_muxer"
 matroska_demuxer_suggest="zlib bzlib"
 mov_demuxer_suggest="zlib"
-mp3_demuxer_deps="mpegaudio_parser"
+mp3_demuxer_select="mpegaudio_parser"
 mp4_muxer_select="mov_muxer"
 mpegtsraw_demuxer_select="mpegts_demuxer"
 mxf_d10_muxer_select="mxf_muxer"