gnulib-tool: Add undocumented option --gnulib-dir. * gnulib-tool.sh: Accept --gnulib-dir=... option. * pygnulib/constants.py (init_DIRS): New function. * pygnulib/main.py (main): Accept --gnulib-dir=... option. Invoke init_DIRS. Expect .git directory to be present in DIRS['root'], not APP['root']. * pygnulib/GLImport.py (GLImport.execute): Use DIRS['root'], not APP['root'].