add exception to allow linking with OpenCASCADE with its own incompatible license (OCTPL)

doc/LICENSE.txt

 Gmsh is provided under the terms of the GNU General Public License
 (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),
-  Netgen (from Joachim Sch"oberl), Chaco (from Bruce Hendrickson and
-  Robert Leland at Sandia National Laboratories) and METIS (from
-  George Karypis at the University of Minnesota) under their
-  respective licenses. You may copy and distribute such a system
-  following the terms of the GNU GPL for Gmsh and the licenses of the
-  other code concerned, provided that you include the source code of
-  that other code when and as the GNU GPL requires distribution of
-  source code.
-
-  Note that people who make modified versions of Gmsh are not
-  obligated to grant this special exception for their modified
-  versions; it is their choice whether to do so. The GNU General
-  Public License gives permission to release a modified version
-  without this exception; this exception also makes it possible to
-  release a modified version which carries forward this exception.
+ The copyright holders of Gmsh give you permission to combine Gmsh
+ with code included in the standard release of TetGen (from Hang Si),
+ Netgen (from Joachim Sch"oberl), Chaco (from Bruce Hendrickson and
+ Robert Leland at Sandia National Laboratories), METIS (from
+ George Karypis at the University of Minnesota) and OpenCASCADE (from 
+ Open CASCADE S.A.S) under their respective licenses. You may copy 
+ and distribute such a system following the terms of the GNU GPL 
+ for Gmsh and the licenses of the other code concerned, provided 
+ that you include the source code of that other code when and as the 
+ GNU GPL requires distribution of source code.
+
+ Note that people who make modified versions of Gmsh are not
+ obligated to grant this special exception for their modified
+ versions; it is their choice whether to do so. The GNU General
+ Public License gives permission to release a modified version
+ without this exception; this exception also makes it possible to
+ release a modified version which carries forward this exception.
 
 End of exception.