Bump license to GPL v3+.

Bump license to GPL v3+.

Fixes future compatibility problems, GNU packages' policy defiance
and general confusion.

Signed-off-by: Han-Wen Nienhuys <hanwen@xs4all.nl>
Signed-off-by: Jan Nieuwenhuizen <janneke@gnu.org>
Signed-off-by: John Mandereau <john.mandereau@gmail.com>
Signed-off-by: Graham Percival <graham@percival-music.ca>
Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>
Signed-off-by: Joe Neeman <joeneeman@gmail.com>
Signed-off-by: Heikki Johannes Junes <hjunes@gmail.com>
Signed-off-by: Neil Puttock <n.puttock@gmail.com>
Signed-off-by: Carl D. Sorensen <c_sorensen@byu.edu>
Signed-off-by: Mats Bengtsson <mats.bengtsson@s3.kth.se>
Signed-off-by: Werner Lemberg <wl@gnu.org>
Signed-off-by: Patrick McCarty <pnorcks@gmail.com>
Signed-off-by: Nicolas Sceaux <nicolas.sceaux@free.fr>