Update mp3lame library probing heuristic.

Since's the mp3lame doesn't seem to advertise itself in pkg-config,
there's few easy ways to detect it.  So instead of forcing users
to specify it in the config file, we'll just assume that the presence
of a working lame binary implies its library is present also
(which works if the user installed from source).
Eliminate compiler warnings.
Remove global audiotools.cfg file.

This should make local installs easier and admins can always
drop their own audiotools.cfg into /etc if necessary.
Fix some of the ReplayGain handling defaults.

This should ensure the tools apply it like they're supposed to
based on the command-line option and config file settings.