The libunistring library and its header files are dual-licensed under
"the GNU LGPLv3+ or the GNU GPLv2+". This means, you can use it under either
  - the terms of the GNU Lesser General Public License (LGPL) version 3 or
    (at your option) any later version, or
  - the terms of the GNU General Public License (GPL) version 2 or
    (at your option) any later version, or
  - the same dual license "the GNU LGPLv3+ or the GNU GPLv2+".

You find the GNU LGPL version 3 in the file COPYING.LIB.  This license is
based on the GNU GPL version 3, see file COPYING.

You can find the GNU GPL version 2 at
<https://www.gnu.org/licenses/old-licenses/gpl-2.0.html>.

Note: This dual license makes it possible for the libunistring library
to be used by packages under GPLv2 or GPLv2+ licenses, in particular. See