Confusing license statements in some Gnulib imports

[Glenn Morris]

Forgive me for being lazy and not reporting to Gnulib.
In the Emacs trunk, eg arg-nonnull.h says:

   This program is free software: you can redistribute it and/or modify it
   under the terms of the GNU General Public License as published
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^
   by the Free Software Foundation; [...]
   [...]
   See the GNU Lesser General Public License for more details.
               ^^^^^^

This also seems to affect c++defs.h, warn-on-use.h, and lib/getopt_.h
(at the _GL_ARG_NONNULL definition).