gnulib-tool.py: Don't default to 'build-aux' for --auxdir. * pygnulib/GLConfig.py (GLConfig.__getitem__): Don't map '' to 'build-aux' for the 'auxdir' key. * pygnulib/GLImport.py (GLImport.__init__): Change conditional to use an empty string instead of None since this is returned when --auxdir is not used.