Fix audiodevinfo, audioinfo, audioread and audiowrite build process * audiodevinfo.cc: added bits_to_format utility function * audioinfo.cc, audioread.cc, audiowrite.cc: make sure to not attempt to include sndfile.h if it is not present

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");