gnulib-tool.py: Allow verifying license compatibility with GPLv2+.
* pygnulib/GLInfo.py (GLInfo.usage): Document the --gpl option.
* pygnulib/main.py (main): Accept a --gpl option. Pass it to the
GLConfig.
* pygnulib/GLConfig.py (GLConfig): Add 'gpl' field and constructor
argument. Add getGPL, setGPL, resetGPL methods.
* m4/gnulib-tool.m4 (gl_GPL): New macro.
* doc/gnulib-tool.texi (Modified imports): Document the gl_GPL macro.
* pygnulib/GLImport.py (GLImport.__init__): Look for gl_GPL invocations
in gnulib-cache.m4.
(GLImport.actioncmd): Output --gpl option when option --gpl was given.
(GLImport.gnulib_cache): Emit a gl_GPL invocation when option --gpl was
given.
(GLImport.prepare): Do license compatibility checking when option --gpl
was given.
* pygnulib/GLModuleSystem.py: Update a comment.