Fixup last commit linker error (Future's algorithm)

.. last commit introduced a compiler error on missing global variables.

   the intention here is to take future's algorithm, embed into a class
   and add wrappers for user charts / datafilters.

src/Core/main.cpp

@@ -76,6 +76,15 @@ QDesktopWidget *desktop = NULL;
 HttpListener *listener = NULL;
 #endif
 
+// future's algorithm (needs to be refactored into a class)
+extern "C" {
+#include "../contrib/voronoi/vdefs.h"
+int sorted, triangulate, plot, debug, nsites, siteidx ;
+float xmin, xmax, ymin, ymax ;
+Site * sites ;
+Freelist sfl ;
+}
+
 // R is not multithreaded, has a single instance that we setup at startup.
 #ifdef GC_WANT_R
 #include <RTool.h>