Different parts of libretools are either the GNU General Public License version 2 or 3; sometimes with, sometimes without the option to use a later version of the license. Which one is specified on a per-file basis; look at the source. In addition to a full copyright and license statement, each file contains a "License:" line to make searching easy. Full copies of the GNU General Public License versions 2 and 3 are included as `COPYING-GPLv2` and `COPYING-GPLv3`, respectively.