remove ambiguity about GPL license version used (specify explicitly v2 or...

remove ambiguity about GPL license version used (specify explicitly v2 or later) in order to fix Debian Bug#617931

doc/LICENSE.txt

 Gmsh is provided under the terms of the GNU General Public License
-(GPL) with the following exception:
+(GPL), Version 2 or later, with the following exception:
 
   The copyright holders of Gmsh give you permission to combine Gmsh
   with code included in the standard release of TetGen (from Hang Si),

doc/gmsh.html

@@ -59,8 +59,8 @@ asked questions</a>.
 <h2><a name="Download"></a>Download</h2>
 
 Gmsh is distributed under the terms of
-the <a href="http://www.gnu.org/copyleft/gpl.html">GNU General Public
-License (GPL)</a>:
+the <a href="/gmsh/doc/LICENSE.txt">GNU General Public License
+(GPL)</a>:
 <ul>
   <li>
     <span style="border: 1px dotted; padding: 3px;">
@@ -163,17 +163,18 @@ gmsh, password: gmsh).
 
 Gmsh is copyright (C) 1997-2011 by C. Geuzaine and J.-F. Remacle and
 is distributed under the terms of
-the <a href="http://www.gnu.org/copyleft/gpl.html">GNU General Public
-License (GPL)</a> (with an <a href="/gmsh/doc/LICENSE.txt">exception</a>
-to allow for easier linking with external libraries).
+the <a href="/gmsh/doc/LICENSE.txt">GNU General Public License
+(GPL)</a> (Version 2 or later, with an exception to allow for easier
+linking with external libraries).
 <p>
 In short, this means that everyone is free to use Gmsh and to
 redistribute it on a free basis. Gmsh is not in the public domain; it
 is copyrighted and there are restrictions on its distribution (see the
-<a href="http://www.gnu.org/copyleft/gpl.html">license</a> and its
-associated <a href="http://www.gnu.org/copyleft/gpl-faq.html">FAQ</a>).
-For example, you cannot integrate this version of Gmsh (in full or in
-parts) in any <em>closed-source</em> software you plan to distribute
+license and the
+related <a href="http://www.gnu.org/copyleft/gpl-faq.html">frequently
+asked questions</a> on the GNU foundation's web site).  For example,
+you cannot integrate this version of Gmsh (in full or in parts) in
+any <em>closed-source</em> software you plan to distribute
 (commercially or not). If you want to integrate parts of Gmsh into a
 closed-source software, or want to sell a modified closed-source
 version of Gmsh, you will need to obtain a different