gnulib-tool: Default to --single-configure. New option --two-configures. * gnulib-tool.sh: single_configure now defaults to false. New option --two-configures. Stop documenting option --single-configure. * pygnulib/GLInfo.py (GLInfo.usage): Document option --two-configures. Stop documenting option --single-configure. * pygnulib/GLTestDir.py (GLTestDir.execute): Update comment. * pygnulib/main.py (main): single_configure now defaults to false. New option --two-configures. * doc/transversal.texi: Update. * HACKING: Update.