Fix 2 minor warning compil (unused variables)

include/boost/polygon/detail/minkowski.hpp

@@ -24,13 +24,13 @@ struct minkowski_offset {
   static void convolve_two_point_sequences(polygon_set& result, itrT1 ab, itrT1 ae, itrT2 bb, itrT2 be) {
     if(ab == ae || bb == be)
       return;
-    point first_a = *ab;
+//    point first_a = *ab;      // seems unused
     point prev_a = *ab;
     std::vector<point> vec;
     polygon poly;
     ++ab;
     for( ; ab != ae; ++ab) {
-      point first_b = *bb;
+//      point first_b = *bb;      // seems unused
       point prev_b = *bb;
       itrT2 tmpb = bb;
       ++tmpb;

polygon/PolyLine.cpp

@@ -339,17 +339,6 @@ int CPolyLine::MakeKboolPoly( int aStart_contour, int aEnd_contour, std::vector<
         for( int ic = ic_st; ic<=ic_end; ic++ )
         {
             int style = side_style[ic];
-            int x2, y2;
-            if( ic < ic_end )
-            {
-                x2 = corner[ic + 1].x;
-                y2 = corner[ic + 1].y;
-            }
-            else
-            {
-                x2 = corner[ic_st].x;
-                y2 = corner[ic_st].y;
-            }
             if( style == STRAIGHT )
                 n_vertices++;
             else