std::round is not defined in some platforms (#30)

Use the C version.

Author: tmpsantos

include/mapbox/polylabel.hpp

@@ -152,7 +152,7 @@ geometry::point<T> polylabel(const geometry::polygon<T>& polygon, T precision =
         // update the best cell if we found a better one
         if (cell.d > bestCell.d) {
             bestCell = cell;
-            if (debug) std::cout << "found best " << std::round(1e4 * cell.d) / 1e4 << " after " << numProbes << " probes" << std::endl;
+            if (debug) std::cout << "found best " << ::round(1e4 * cell.d) / 1e4 << " after " << numProbes << " probes" << std::endl;
         }
 
         // do not drill down further if there's no chance of a better solution
@@ -175,4 +175,4 @@ geometry::point<T> polylabel(const geometry::polygon<T>& polygon, T precision =
     return bestCell.c;
 }
 
-} // namespace mapbox
+} // namespace mapbox