gnulib-tool.py: Fix 'Undefined variable' errors. * pygnulib/constants.py: Import codecs. * pygnulib/GLConfig.py (GLConfig.setLibName): Fix local variable reference. * pygnulib/GLFileSystem.py (GLFileSystem.lookup, GLFileAssistant.update): Fix local variable references. * pygnulib/GLEmiter.py (GLEmiter.po_POTFILES_in): Fix reference. * pygnulib/GLTestDir.py (GLMegaTestDir.execute): Define missing local variables.