Add qhull source code and license file

extern/qhull/COPYING.txt

@@ -0,0 +1,38 @@
+                    Qhull, Copyright (c) 1993-2012
+
+                            C.B. Barber
+                           Arlington, MA
+
+                               and
+
+       The National Science and Technology Research Center for
+        Computation and Visualization of Geometric Structures
+                        (The Geometry Center)
+                       University of Minnesota
+
+                       email: qhull@qhull.org
+
+This software includes Qhull from C.B. Barber and The Geometry Center.
+Qhull is copyrighted as noted above.  Qhull is free software and may
+be obtained via http from www.qhull.org.  It may be freely copied, modified,
+and redistributed under the following conditions:
+
+1. All copyright notices must remain intact in all files.
+
+2. A copy of this text file must be distributed along with any copies
+   of Qhull that you redistribute; this includes copies that you have
+   modified, or copies of programs or other software products that
+   include Qhull.
+
+3. If you modify Qhull, you must include a notice giving the
+   name of the person performing the modification, the date of
+   modification, and the reason for such modification.
+
+4. When distributing modified versions of Qhull, or other software
+   products that include Qhull, you must provide notice that the original
+   source code may be obtained as noted above.
+
+5. There is no warranty or other guarantee of fitness for Qhull, it is
+   provided solely "as is".  Bug reports or fixes may be sent to
+   qhull_bug@qhull.org; the authors may or may not act on them as
+   they desire.