* Fixes an obviously long standing bug with the -a/--apply options.
* The FFmpeg used to build bs1770gain needs to be configured with option --enable-libmp3lame
  (cf e.g. https://stackoverflow.com/questions/5021120/ffmpeg-mp3-conversion-failed.)
* @Debian: The configure.ac script is adapted following your proposal in order to support building on <i>Hurd</i>.